Just about every art store I have been into has had displays of extra/leftover pre-cut mats and mounts. Give that a try.
I personally use mats from http://www.nielsen-bainbridge.com/ that I buy from http://www.aifriedman.com/ in NYC. I find their prices reasonable - 11x14 pre-cut 100% museum 8 ply mat & 4 ply board is $8. You have to call them for these as they are not on their website yet.