I like Michael's, as all have said, too. Nice frames.
And if you want to do it yourself, which is what I normally do for just hanging pics around the house, any art store should have matboards (Pearl Art does), and they are $5-10 depending on if they are acid free, the color, etc, for a 30"x40" size (makes about 6 11x14's). Then you can get a matboard cutting kit, follow the directions, and mount to whatever size, shape, etc you want (kit is about $40 total, usually. More for extra accessories).
Also, for really cheap frames, Walmart sells 10x13 and 11x14 frames for $2.50 each locally, usually. They are cheap cardboard backs to hang them, but great for giving someone a little present (virtually free printout from print, matboard is less than $1/pic, $2.50 frame).