Ok, so these are decidedly a step (or three) up from the reproduction "Chat Noir" posters that you hung in your dorm room in college. Elbé, in St. Germain, has been selling vintage wall art since 1974 and is the place in-the-know Parisians go to get it. Be prepared to spend hundreds on a high-quality poster. The scale is big, and the colors truly pop off the paper.
Elbé also has a great collection of original 18th-century drawings of mushrooms, shells, plants, flowers, and the like. At prices of 40-200 euros, these demure works are more affordable than the splashier liquor and tourism ads. (My girlfriend and I bought four for our Brooklyn apartment.)
Everything here is well-archived, and they're used to taking requests for specific posters. Whether or not you're a collector, the comprehensive website is a pretty addictive place to do some browsing, too.