Most rare and valuable vintage posters are sold by auctions houses. A few auction houses are specialized in vintage poster auctions.
All auctioneers charge a fee for the items they sell. This ”buyers premium” is usually between 20% and 25%. If you want to bid online, you have to create an account on their websites before.
Have a look at my choice of selected poster auctions worldwide.
On the other hand, in big cities you will find poster galleries or poster dealers. Most of them have online-shops, too. My collection is based on a find of seven vintage airline posters in a local antiques book store...
And last not least, with some luck you may find rare vintage posters at online auctions like ebay.com or other online marketplaces worldwide. Search by name of airlines, destinations or artists and you will get offers worldwide. But be careful: it’s always a risk to judge about poster condition viewing on a computer screen only.
Believe me, it needs a lot of time to check out thousands of offers every month, but some of them are real bargains.