These are a few of the available modules that will work in the 7U.
http://www3.towerhobbies.com/cgi-bin...?&I=LXRTT5&P=0
http://www3.towerhobbies.com/cgi-bin...?&I=LXYPW0&P=0
http://www3.towerhobbies.com/cgi-bin...?&I=LXXCY3&P=0
http://www.horizonhobby.com/Products...odID=SPMMSFUT7
http://xtremepowersystems.net/prodde...hp?prod=XPS-F2
http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=7381&Product_Name=Corona_2.4Ghz_Futaba_Module_&_Rx_(V2_DSSS)
http://www.hobbycity.com/hobbyking/s...W/_Module_&_RX
I think the six channel transmitter just has a crystal rather than a removeable RF module. These are supposed to be able to convert virtually any transmitter to 2.4 gHz. The second one uses the trainer cord jack. Not sure about the other one. Again, these are just examples and there may be many more options that I am not aware of.
http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=9770&Product_Name=Corona_2.4Ghz_DIY_Module_&_RX_(DSSS)_
http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=8354&Product_Name=X8D_2.4GHz_trainer_port_X8-hack_module_(PPM_Only)
Once you choose a brand for the transmitter end, only that brand of receiver is compatible. Also, there can be various versions within a brand (one of the Corona parts mentions this) that are not compatible.