When first getting involved with a new culture, especially one that promotes a DIY attitude, one of the first questions that people tend to ask is where to find material. I don't mean source material. Google images can take care of that for you. What I'm talking about is the physical material to make your objects. While it would probably be easier to list the places that you cannot find material, I'll try to share some of my favorites here. Of course, not all of these are my ideas. Some of the specifics that I recommend were first recommended to me by somebody else. I'm going to try my best to cite original sources where I can.
You can buy just about anything in the world online, if you know where to find it.
One of the things common amongst many steampunk items are latches and clasps. These hardware bits, especially the more interesting ones can be a bit difficult to track down, but Katie and I have been able to find most of the things we're looking for at Hardware Elf. Their website is very basic and it's very clear that they're a very small company, but their prices are reasonable, and they're one of the few places that I've found to have what is referred to both as a box latch and a swing latch. This is the type of a latch with a post on one side, and on the other, the post goes through a hole in a plate, and a pin rotates down to hold the whole thing in place. They also have the same style of dial latch that is on the leather journal that Katie and I will be using for our wedding, in addition to many other kinds of hinges, latches, and locks.
When it comes to garments, the easiest way to drop a little bit of steamy flair in is to replace buttons with gears and clock innards. Treasure Cast has die cast buttons, pins, and similar items from a number of different cultures. They're a bit pricey on a per piece basis, until you factor in that these buttons are die cast, incredibly accurate, and very high quality. The buttons on the coat for my steampunk character Morrison (who I'll introduce in another post) came from here, as well as the buttons that will be on the coat for my Steampunk Dr. Who, the buttons on the coat for my wedding, as well as the stick pin that I will be wearing in my lapel.
While many items on the site are overpriced, Etsy can be a great place to find things that are hyper specific that you can't find anyplace else. If you can't find it there, you can buy just about anything on eBay or Craig's List, including somebody's virginity.
For a more hands on experience, visiting swap meets and thrift stores is a great idea. The secret here is not to look for a bag of gears (they'll be marked up in price), but to look for the types of things that have gears inside of them that you can take apart and get the gears out of. In addition to gears which can be useful to have around, you can also salvage electronic components for future electrical projects. My only warning about electrical components is to be very careful with dismantling them, as anything with a capacitor may be carrying a residual charge.
During their panel on gadgetry at the World Steam Expo, the folks from Outland Armour shared a whole bunch of ideas for sources as well as specific product recommendations. A lot of their work is done with a product called Sintra. This is a closed cell expanded PVC foam board. Sintra is a brand name, so you may find the same product under many names. It is very easy to heat, form, cut, paint, join, ect. They also make a lot out of orthopedic polypropylene. They say that they were very lucky to find a source for the material in their hometown because the material can be tough to find. It's a plastic with a foam already glued to the inside and is used to make medical devices like neck braces. With care, this can be formed for things like forearm braces, which can then be used to build all kinds of gadgets upon. Among the other specific products they recommended are the entire family of Rustoleum products (specifically, their metallics), Rub n Buff, Krylon Gloss Clear Coat, brass upholstery tacks and nails, WED clay, Bondo, Plastillina, and others.
The secret when it comes to finding items and supplies for gadgets and toys is to not think "What is this?" but to think "What can this be?" or "What can I use this for if I take it apart?" Take a look at the jet pack from Outland Armour's Steampunk Boba Fett. According to Danny from Outland, the top of the center chamber (the silver pointy bit) is a tulip bulb planter, the outer chambers (with the rounded tops) were picked up at a military surplus store and originally held rockets, and if I remember right, the center chamber is mostly made from a large, cheap, plastic thermos. Looking at the wrist mounted flamethrower, you can see the same type of trunk latches that you can pick up from Hardware Elf.
When it comes to clothing, if you're going to make it yourself, there is no better source of fabric than the home decor department. The fabric that you'll find here is much thicker and heavier than standard fabric, which is great both for things like coats and kilts as well as for other garments that require a bit more heft, like corsets. In the Detroit area, the best place that I can recommend is the Fabric Warehouse in Warren. This family owned place stocks a wealth of fabric and trim, all at great prices. Nothing sells for more than $10 a yard. The owners and incredibly nice and helpful, the material is very high quality, and their cuts are rather generous. A few weekends ago, we stopped by and ended up getting about an extra 3/4 of a yard for free. Why? They don't sell less than a yard, so if they get to the end of a bolt and there's less than a yard left, you get it.
There's a number of companies that produce patterns which may be useful. While the usual suspects have a few patterns that are appropriate (Simplicity has a new dress and apron combo, a men's and women's matched driving coat set, as well as what I lovingly refer to as the Sherlock Holmes coat), the more accurate and less popular patterns will be found elsewhere. Period Impressions features a large number of great patterns for both men and women. Their patterns can be found from a fairly large number of distributors. A different variety of patterns can be found from Reconstructing History. However, because of the care that they put into ensuring that their patterns are as accurate as possible, they are fairly pricey and require a bit more garment construction knowledge.
I could go on for quite a while, listing potential sources for material, but the truth of the matter is that nearly anything can be used if you look at it the right way. I know more than a few costumes that have been put together by people who didn't by anything special, they just looked at what was already in their closets. If I come across any specific sources that are particularly excellent in the future, I'll certainly share them. Hopefully, this keeps you going for a while.