(Inexpensive) Leather
Western Hide and Tanning Co
"We can offer a special selection of 12/14 ounce leather and split it for you into 9/10 ounce weight. Price is $60.00 per side - minimum 3 sides. Thanks, James Cox - Western Hide and Tanning Co. - Hamilton, Ohio USA
1-513-867-8699 or 867-0802 for fax."

The above are vegetable tanned hides, which means they will harden well using ordinary hot (or if you don't mind shrinking), boiling water. - Brian Price

Rivets
RJ Leahy company

These are the cheapest and best assortment I've found - Krag

Carbon Steel
Admiral Steel
They have knife/sword steel as well as carbon sheet steel that could be used for armoring. Brian Price, author of the soon to be published book on armor construction, recommends the 1050 as being close to medieval armor steel, and relatively easy to work. Not for beginners however. Stick with mild steel unless you've got experience.