depending on the size of the poster, sometimes poster frames come in standard sizes…you just buy, put the poster in, et voila. Try the U-frame-it location at 3237 Calif. Ave. SW. Check the sales rack and sizes of pre-made frames…they might have something or could give you suggestions where to go.
I actually framed something myself once(not my forte, either) at NW Art and Frame, and while not terribly difficult, was a bit daunting, and the person helping me truly didn’t realize that I had no idea what he was talking about. He wasn’t very patient, and I never attempted to do it again.