Not sure what part of the country (or world) you live in, but most major craft stores such as Micheals, Pat Catan's, etc., carry pre-cut mattes in most standard sizes (11x14 mattes cut for 8x10 photo's, 5x7 mattes cut for 4x6's, etc..). Additionally, most of these types of stores also carry the tools to cut your own mattes, ranging from very basic to quite elaborate. Personally, I cut my own mattes now...most pre-cut double mattes cost around $5 a piece...when I buy the matte board (depending on type and style of course) I can cut my own for around $1 a piece. I use a $20 Logan cutter with a home made cutting board (a piece of laminated wood left over from some counter top with a cheapy carpenter's square screwed to it). I also have a Logan straight cutter which I use for cleaning up the edges since I can't cut straight with scissor's if my life depended on it! LOL!!! Seriously, if you need a lot of mattes, it will save you quite a bit of money to learn how to cut them yourself and it's not really that hard to do at all.
One little tip to save yourself a buck or two if you should decide to cut your own...check with craft stores that do actual picture framing as they sometimes sell their cut out's and scrap pieces for pretty cheap. You can often get some really nice quality matte pieces for a lot less then you'd pay for the stuff by the sheet. Two of the places near me sell 11x14 blanks for $1 a piece and some of it is that stuff that goes for $50 a sheet (I usually get 6-8 11x14's out of a sheet then I use the 8x10 cut outs to make smaller mattes...5x7 and so on)....ya can't beat a bargain like that!
Now as far as "qualities" go, generally matte board comes in two basic varieties..."Ph balanced" and "acid free". The acid free stuff is usually museum/archival quality, but also costs -A LOT- more...as much as $30-$50 a sheet or more, where as I get the basic Ph balanced stuff from my local crafts store for $5 a sheet and I've never had a problem with it. I usually stick with the Cresent brand, however I've used Banebridge from time to time when I find it on sale. The main things to look for when buying pre-cut mattes is nice clean cuts and sharp corners without any "over-cut". It's also worthwhile to check the corners of the board to make sure no one's dropped them and mushed an edge or something.
Alrighty, I hope this helps!
Bright Blessings & Gentle Breezes,