I have been waiting to find new themes for my 8800, though I do like one of the current ones I have, but I have yet to see any available for the phone. Does anyone know where I can get some that might not have been included with the T-Mobile 8800?

Wirelessly posted (8700c: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0) BlackBerry8100/4.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102)

Plazmic or RIM has not released any yet. Probably in a couple of months or so but this merely speculation.

There is no way to make 8800 specific themes as there is no Theme Builder released for it.
Some 8700 themes made in Theme Builder may work, but they're not officially supported to it's a crapshoot.
These are all the RIM made themes available for the 8800: How to Add Vendor Locked Themes - BlackBerryFAQ