Just remember, you get what you pay for. If you just want cheap mats without any thought to longevity then go for the cheapest junk you can find.
If you want a good archival mat then they will cost more. All of the major manufacturers carry the good stuff as well as the junk.
I prefer Bainbridge and Rising over the others out there.
A place like United Manufacturers will sell you mats and ship them to you.