For belts (if I can't find it at the salvation army/good will store ^^
, I usually use black vinyl from the fabric store (or whatever colour needed). It's perfect for belts and you can cut it to any shape you need, of course.
I've only made one belt buckle and it didn't turn out too swell, so I always thought that next time I'd use friendly plastic. They sell it at most hobby stores, and it's just a piece of plastic that when you dip it in hot water you can mold it (after it cools off it's solid like plastic again). I used it to make some of my jewelry for my lina costume, and what I did was cut cardboard into the right shape, then melt the friendly plastic over it. It gave it a nice hard feel and look to it. It comes in gold and silver colours, too.