Sometimes I get the really basic wooden frames with the thin glass, 8x10 and 11x14. Not bad to give away as inexpensive gifts with prints inside, especially when I cant buy something. I have also picked up a cool super minimalistic plastic panoramic frame, its 4 inches by 12 inches. These items aren't really 99 cents, but more like $1.59 to $2 even though they are in the 99cent store.
I would get mats if they have them in my store, but they dont. Thats a cool 99 cent store that you have that would have something so specialized.