Expansys.com and Mad Monkey Boy's Gadgets (www.dweiniger.com) both sell it.
Not much drama at all to get it working on Cingular-- I used it on both Cingular and T-Mobile. The phone will pick up the text message server address from your SIM and you'll need to enter the approp. Internet settings in the connection manager to get online with IE and e-mail (not too hard). I did not try the MMS client config, but it looks straightforward. MMS is generally the only thing that can be cranky on unlocked phones, the rest is a piece of cake.
-------------------- Lisa Gade Editor in Chief, MobileTechReview