eBay is pretty much your only EASY bet aside from other game collecting forums (digitpress.com comes to mind). Craigslist hardly ever gets specific video game memorabilia like that, as I'm sure you know... If you want to get any posters of the Japanese box art (or from games not released in the USA) Yahoo! Auctions Japan is a great resource, but you would have to pay a middleman service (Celga is the most suggested but I've had good results using Shopping Mall Japan). Video game/anime conventions also sometimes have booths selling posters, but those are a lot harder to find.