I got a U.S. Divers belt buckle from eBay recently. I got mine for $40, but I've seen them go for upwards of $75.
If anyone's interested in trying to make their own buckle out of something like aluminum, here's a blueprint. You'd probably want to use 16 or 20 gauge material.
The dotted lines indicate a bend, except the lateral dotted line on the buckle base, which indicates an optional rolled groove (that my buckle has). I'll try to find a picture of that.
The PDF template is intended to be printed at 100%. Enjoy!