Be patient, and you should be able to. Check Ebay regularly, avoid dealers, as long as you don't rush, you will find them at a reasonable price.
T
Please ignore the comment "avoid dealers". T lives in a different world to the rest, the fact is he is descended from a centuries old family of 'dealers' aka Art gallery owners, people who made money selling other people's art.
'Dealers' aren't the bogey man, 'dealers' are unlikely to pass on a venereal disease and many 'dealers' are avid collectors so are happy to discuss your habit with you and in many cases help advise you too. Sure, there are scoundrels out there (as there are is in in every walk of life) but the fact you are here (in this forum) will help further your education on who/what/where and when.
A dealer (whether he be full time or part time) has spent money buying the poster (costs that may include postage getting it, petrol picking it up, perhaps even Post Office box fees), they will then spend time and money advertising it (if they have their own website there are on going expenses), they also pay for the packaging, take it to the PO etc to then get it to you.
A dealer has the poster you are looking for, there is no waiting, no auction bidding war and generally no problems with their reputation (again, use this forum and other research) - you want the poster, you buy the poster you get the poster, the maths is the only thing you generally need worry about.
So sure, do your research and you should be good no matter where or from whom you buy from.
BTW - by definition a 'dealer' is someone who buys, sells AND/OR trades in a commodity - that probably makes almost everyone here (including T) a 'dealer' in some form or other