I framed all mine myself. Either picked the frames at Ikea or Target for $20. The problem with any frames you buy is that some posters aren't 27" x 40", which is the dimension of a standard one sheet. Some are 1/2" either tall or wide. You just have to be creative on how you frame it. Like my Leon poster was short about 1/2" in width, so I just took some black electrician tape and placed that on the edges of the matte/background. You mostly run into these problems with posters before 1980 or from outside the US.
allposters has a good selection. it really comes down to are you buying art for looks or for value. All my posters are reprints, so they range from $10 to $25. I know some people that are really anal and only want original, or attempt to find lightbox prints (printed on gloss paper on both sides to allow light to pass through).