YO, ok first I hope you're an avid "online" shopper, or have a lot of thrift / junk stores around.
For my self I salvage lots of stuff from old construction sites. Though for a list of stores...
Aether stores
www.Ebay.com http://www.metalsupermarkets.com/http://mad-salvager.squarespace.com/new/Brick and motor venues
Home Depot
Lowe's
Flea Markets
Other
A good place / time I find to get stuff is "Large trash pick up day". Check your city's local "web page" and see when it is for your area. there is almost always one going on where i live so i can just drive there any time of the month and Loot "junk" (Poor fools don't know what their throwing away).
You May also find this site useful
http://steampunkworkshop.com/lab-links.shtml