Well, dress belts are usually 1-1/4", and casual belts are usually 1-1/2". You can get obscenely large gun belts (2+"), but most of the good makers do 1-1/2". I just recently got mine from
The Beltman. Strong, stiff, double-layer bullhide reasonably priced and took only a couple of weeks. He attaches his buckles with screws precisely so you can swap them out for your own buckles.
At the worst, I figure you could send him a pic of your current buckles (taken next to a ruler or tape measure, I guess) and he could let you know if that'll work with his products.
Good luck!