Hey, welcome along
If you are keen on vintage toys, I assume you are after vintage poster - 77, 80 and 83. Although font discount the 97 special edition poster, cracking art work, better than a lot of the 70's and 80's stuff (IMO)
There are a lot of things out there to buy, even Evilbay sells this stuff (or people on evilbay), but I would be very wary on that front as there is a lot of crap and missleading sales up there for posters.
A few good online dealer sites that i have used before are:
http://www.simondwyer.com/ - London based chap, very easy to deal with and good value and good stock
http://uk.movieposter.com/ - this is a US based company that ship world wide, they have good stock and are very reputable but their prices are a bit heigher and no negotiation as a rule as it is all Internet based.
http://www.moviepostersdirect.co.uk/ - not vintage but this site normally has a full set of special edition posters up at a fair enough price.
Lots of this to watch out with vintage posters, especially SW once that are very copied and faked, not least because some of the copies are from the 70's and 80's where some of the liciencing allowed companies to reprint the movie posters legitamatly.
You will also want to think about what types of posters. Ther are three main types, UK quad, US One Sheet and Half Sheets, there are also Inserts but these are the main ones. Within that there are advances, and different styles etc, quite a lot out there. A few quick tips about poster from back then:
1 - UK quad should be 40"x30"
2 - US One Sheet should be 27"x41"
3 - almost all these posters will be folded not rolled as this was how they were shipped to cinemas back in the day
4 - pay very careful attention the the markings and data printed on the bottom. It should tell you the style and print code etc, but these are well faked.
5 - unless you are using a very trusted online dealer (and there are a few, way more than I listed) you want to check the poster out in the flesh.
6 - have a lot of funds some posters go for thousands

7 - be sure to post images of any you get here, I love to see vi take posters (I only own 1 vintage Empire half sheet, but I do have a full special edition set, which is very nice and a fraction of the cost of a vintage set)
Best of luck.