the TBP12 is available from any LIne 6 dealer.  even if they don't have it in stock they can order it for you if you ask them.  here for example: http://www.dv247.com/microphones/line-6-tbp12-channel-wireless-transmitter--83632

I believe I've seen the question asked before, and the 6 and 12 channel guitar and mic wireless units from L65 all use the same 12 channels (the 6 channel units use the 1st 6 channels from the 12 channel systems) so it's all compatible,  but remember that the connections may be different so if you're using for guitar then it would be best to use the correct transmitter from the G50 or G30.

Rowbi