To frame original work, I would recommend going to an art and crafts store like Michaels (
http://michaels.com/art/online/home). They do custom framing for unique sized art. They also have a large stock of frames for typical sized work. You can often get 50% discounts there using coupons or going during a sale.
If you would like something more simple, just for finishing purposes, I would recommend buying your materials from Home Depo or Lowes. You can get molding or trim by the foot and build your frame. I often use lattice stripping and nail it to my framed canvas. Then paint it white or black, just to finish off the painting. It is simple, yet effective.
good luck,
Joe