I've seen it at Orchard Supply Hardware, Home Depot, Lowes, most hobby stores. Brass sheet, that is.
Brass fillings can be had at your local key makers' dump box.
Another source for brass is to check with a najor auto supply store, or a place like Grainger's and get some brass shim stock. It comes in a roll and you can get it in .002", .003", .005" etc in thickness.