I'll second Frame Destinations. Buying from it for a little over a year and I am a pretty satisfied customer.
Occasionally, FDI does a special on ten packs on pre-cut mats on select color/sizes. If you can wait for it, it's an excellent deal. If not, by grouping your order you can save enough to make paying for shipping a worth while expense. Initially, you'd think their shipping is a bit high but you'll understand why once you get your package. (trust me, it's worth it....)