Texas Knifemakers Supply is one place that I have purchased from a few times years ago and without any problems.
Here is the link CLICK HERE...click on Kydex and Leather on the left hand side of the page.
I didn't buy Kydex from them but, I did buy other knife related items.
They have been around for a long time and are an established company.
They do sell Kydex in 12" Square and 12"X24" sheets.
Some other members may have cheaper sources of supply so "hold off" ordering until more members post comments in your thread.