At any rate, there are many outlets for bulk quantities of quality mounting and matting materials to be found online.
I recommend you get samples from a variety of suppliers that seem to have product you would be interested in, and compare prices after a close examination of the samples.
Hobby Lobby is where I go. They can custom cut a mat for pretty cheap. I had one cut for a 20x30 picture in a 24x36 frame for about 7-8 bucks if I remember right.