One source for kydex and other knifemaking supplies is "Texas Knifemakers Supply" 10649 Haddington, Suite 180, Dept TK, Houston, TX 77043: (713)461-8632. As I recall they have different thicknesses as well as overall sizes. They are real nice folks down there. And I'm sure they can help you.
Try www.blade-tech.com . I have had excellent experiences with them. Besides, most of the kydex you usually find in the other supplies houses is 12"x 12" wht would be a bit tight for a 15" khuk depending on how you designed your sheath.
You can also get the screws and eyelets from them and, if you don't have one already, their pressing rubber is of exvcellent quality.
If you need more tips on making your sheath, just e-mail me.
Rob, I found Texas Knife supply's web site, www.siteblazer.net/texasknife
Also Texas Knifemakers Supply's stock sizes for Kydex is 12"x24" the two thicknesses sell for $4.25 and $6.00 a square foot.Found a toll free phone number too (888)461-8632. Have fun