There are a variety of choices available... What type of transmitter and plane do you want to use it with?
The GWS R4P (
click here) is 3.6g - 4.4g
The GWS R4N (
click here) is 7.6g - 8.0g
Hitec's Feather 4 (
click here) is 8.0g
Hitec's Micro 555 (
click here) is only 15g without the plastic case
Hitec Electron 6 (
click here) is only 13g without the plastic case
FMA M5 Sub-micro (
click here) is 8.5g
FMA's entire Quantum series (6 & 8 channels) (
click here) are 14.1g (0.50 oz)
The lightest Futaba Rx on 72Mhz (the R122JE) is 17g and only 2 channels.
I doubt JR or Airtronics have one in your weight range either (I haven't checked).