Why does it have to be on the wall?
Get a piece of plywood that is pretty smooth.... put the poster against it with a piece of plexi glass over the top of it.
Then get something like an old trunk, two matching side tables, the frame from an old coffee table or just the old coffee table itself....
and just put your new 'poster table top' on it.
You can enjoy your poster more when it is right in front of you as a funky art piece. )

But.............. if your dead set on framing it....
go to yardsales/auctions and estate sales ... find a frame that already has something that size in it .... tear it apart and put your in there instead.
Will save you considerably on the cost.