Tandy Leather has just what you want, I believe:
Lightweight Cowhide Straps
The description says these are 4-5 ounce thickness, as opposed to the "normal" 8-9 ounce weight for leather strap material. I think they are overpriced at $9.99 for a 1/2" x 50" strip. I bought a couple of them when they were on sale a few years ago, though.
Generally speaking, I think you get a better deal from Crazy Crow. They have 1/2" x 48" straps on sale right now for $1.93. Here is a link:
Pre-Cut Strap Leather
The leather from Crazy Crow seems more supple and less stiff than the Tandy strap leather, to me. I have made several belts from the Crazy Crow leather and they hold up well.
It is noteworthy that the Tandy straps are cut 1/16" undersized, for width, so they fit in the buckle more easily. So the 1/2" Tandy strap is actually 7/16" wide. The Crazy Crow straps are full width.
All of the above refers to leather belt "blanks." If you want one dyed and finished, with the buckle installed, I'm sure some of the leatherworkers here will be able to fix you up.
Best regards,
Notchy Bob