Samsung Galaxy S2 — From where can you purchase a phone compatible with AT&T's Network?

I am trying to determine the best vendor/reseller as well in my thread, too: https://www.nextpit.com/en/android/forum/thread/431317/Samsung-Galaxy-S2-Question-about-where-to-purchase-unlocked-S2-and-using-it-in-the-U-S-some-other-general-Q-s-about-the-phone

Anyhow, I've narrowed it down to the best three choices for those like us living in the states with AT&T: Amazon.com, NewEgg.com, and ExpansysUSA.com. The question now remains, which one would be the best out of those three? Amazon has many vendors that it sells. Some has gotten quite negative reviews. I've used NewEgg in the past, and they provided great customer service with all my computer parts. However, one must remember that all these unlocked phones are outsourced, so the quality may not be the actual vendor's fault, but rather who they got it from. Last, but not least, Expansys seems to have gotten decent reviews too.

@Jordan...

That's some excellent advice. It's super important to read vendor reviews, especially considering what a big investment it is to buy an unlocked SGS2.