There are a ton of options to replace a Tx battery.
Have you considered a 3 cell LiPo? I charge mine about once a month and fly a ton off it.
Or, LiFe packs are a great option.
Search around the internet a bit and look at what is available at HobbyKing or Hobbypartz...
You don't have to have an Airtronics battery in an Airtronics radio; it's about the voltage and mAh capacity.