Depending on where you live you shouldn’t have too much trouble finding a comic book shop or card shop or dedicated game shop that would have a copy of something on the shelf. There isn’t really a national game store franchise. It is mostly just some guy with a store or maybe a regional concern with four or five shops.
My local shop usually gets one copy for anyone who’s a regular who asks and one for the shelf when a new version comes out. Typically though he doesn’t replace that till Christmas shopping season but will order one for you with a small deposit. The markup on these games isn’t very high so you probably aren’t going to go to one shop and find a copy of everything.
Toys R’ Us probably has copies of ‘42 and maybe a decent chance of Guadalcanal or a less decent chance of BOTB. I’d think it pretty slim to find a copy of Pacific or Europe at Toy R’ Us given the price point.
Try your yellow pages under hobby, or games, or comics. Also you might try something like www.comicshoplocator.com and give your closest shop a call. You might also try something like this Days of Wonder Store Locator. Someplace that sells games like that is likely to have a copy or two of Axis and Allies.