Some posters DO come in standard sizes, measure them up and see what they are, then shop your local Wal-Mart or crafts store, maybe even Target, to see if they carry that size. If you can't find a standard size frame ANYWHERE, most likely it has to be custom framed. Now comes the fun part.
Custom framing can be done at a number of places. Ever seen an art store in your local mall? You should have 1-2 at nearly every mall. All of these places will do custom framing. You can generally talk them down as well. I recently had a 24x32" poster framed at one of these places. The first place quoted me $129! I was pissed as hell since it was only a $25 print, so I went downstairs to the other place. They quoted me $85. I talked them down to $65 and had them frame it in a black metal frame which was glossy. They also used real glass, which you want to make sure they are doing before they frame it. Generally they can also do glass with no glare for a bit more of a cost.
There are also individual shops, like Fast Frame, who do custom framing and that's all they do. Because these people have high overheads, their prices are going to be a bit high as well on the framing, but it's all going to be a matter of how good of a talker are you, and how much they quote you initially. Some places jack up prices like none other, others simply want to make a buck. Shop around and see who offers the best prices, and let them know, that if they treat you right today, you have additional work you'd like done from them. Hope this helps.
dm