For several years, I have been getting my frames from FrameDestination, which someone else already mentioned. They have low prices and shipping is relatively inexpensive. A big plus is their very attentive customer relations people; they have called me on the occasions when I forget to specify "no points" for a frame because they noticed that is the way I usually order them.
They will make any size you want for a small set-up fee, including frames, mats, and glazing. For a long time, FD just offered fairly simple black wood and metal frames, but now has some fancier frames at reasonable prices. I especially like their simple stained wood frames for many of my prints.
FD provides online instructions on how to mount photos using hanging tissues and most everything you need to know to frame your prints.
I prefer to use 3M's 568 Repositionable Adhesive to mount my photos to GatorFoam so they don't develop waves and then spray them with PremierArt Print Shield so they don't need to be covered with glazing, but that is another story; Frame Destination does not provide any information about that.