A number of people recommend the Spektrum DX6i transmitter for use with tricopters. It appears to go for around $110 shipped and, as the name implies, has 6 channels.
Further research indicated the Turnigy 9x is a better buy. It is only $54 from hobby king and comes with an 8 channel receiver. This transmitter utilizes an atmel microcontroller and has aftermarket er9x firmware available. Additionally, it is very popular to mod (backlight, better radio module, etc.)
Hobby King also provides a newer Turnigy 9xr. This model aesthetically didn’t appeal to me as much. Apparently it also has less internal space for putting in mods and isn’t quite as ergonomic.