The trick to framing is in simplicity. You don't want anything taking importance away from your painting/photograph/memorabilia, so why pick a huge, ornate frame? The beauty of this philosophy is that simpler frames also happen to be less expensive.

Here are a few places to try:

IKEA (http://www.ikea-usa.com)
IKEA sells a lot of very basic frames. They come with glass, pre-cut matte boards, and you can get them by the truckload. One plus for IKEA is that they offer a lot of square frames and mattes, which American framing companies never seem to make.

Dick Blick (http://www.dickblick.com)
Most art supply stores have a framing section, since most of their clients will need to present their artwork in some shape or form. Mix and match pre-made frames with pre-cut mattes for a custom look. Unless otherwise specified, Dick Blick ships all of their frames with plexiglass instead of glass, so read carefully before purchasing.

PictureFrames.com (http://www.pictureframes.com)
A whole host of online framing companies have cropped up in the last few years. PictureFrames.com has a nice interface that allows you to create custom frame and matte sizes for excellent prices. You can also choose from a large pre-made selection.