If you're looking at used instruments, archtop.com has a good reputation and usually a good selection. Much safer than buying from eBay or craigslist.
What Steve said about adjusting your expectations when coming from the world of flattops is really important. You'll have to change your technique to coax the best sound out of an accosting archtop. The standard campfire strum just doesn't usually work well. As a starting point, I suggest trying one of those small, heavy, teardrop shaped jazz picks. I like the Pro-Plec 1.5mm picks. It will help you focus on individual strings, even when chording.
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