I have printed many 12x18 prints. I like to go larger, but 12x18 is a nice, affordable size. Michael's had been mentioned. If I did a lot, I would probably go the route of Teamspeed. Michael's is not cheap, but I don't sell matted/framed prints often. They are usually a gift from me, as most clients want to choose their own frames.
The key to Michael's for me: They are ridiculously priced & they know it, so they often have "sales". I can usually get a bogo 2/$50, 2/$80. If you purchase they frame there & take the print with you, they charge me $30 to mount. Comes with a paper backing, hanging wire, & tabs at bottom so it hangs straight.
This puts me around $50 for a 12x18 matted & framed. If I did any kind of volume I'd have to reevaluate my approach.