They usually have them at Wallmart (if you live in America) if not there, probably Hot Topic and Wallgreens. Best buy? Bed bath and beyond? The Wizarding World of Harry Potter?
Suncoast shops are a good bet if they're still around. Also if you're ever at Universal Studios Hollywood, there's a sci fi shop on City Walk that tends to have good film merch as well as several other shops on the block. However, even though you said to not suggest a website, the ultimate best way to get HP posters and cheaply too is through Amazon. I got my amazing Hermione Half Blood Prince poster for $11 and that included shipping and taxes :)