Thinking of getting a Lumia 520 on AT&T GoPhone

I own an iPhone 5 that I like, but I'm considering getting an AT&T GoPhone Lumia 520 to play around with since I'm VERY curious about Windows Phone, but don't want to jump in head first yet. It's $100 bucks...can't really go wrong.

Question is...can I just buy this phone without signing up for a plan? When I look on AT&T's site, it doesn't look like you can do that. All I'd want to do is swap my SIM from my iPhone to the 520 when I wanted to use it. Are these carried in any local stores (AT&T, Best Buy, etc)?