I just found this add. All Electronics is selling the add-on vex transmitters for $29.95. http://www.allelectronics.com/cgi-bin/item/JS-6/search/6-CHANNEL_TRANSMITTER_AND_RECEIVER_.html
This is a great way to pick up extra transmitters. 
American Science and Surplus has them for $24.95.
You can also get then brand new on Ebay for less than $20.