I went to lipo in my Tx this year too,, it's a great upgrade, don't know why I waited,, well I do know, I didn't need a new pack yet.
Charging it once a month depends on how much use,, but that's about average for me too.

I got this one http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6955&Product_Name=ZIPPY_ Flightmax_2500mAh_Transmitter_Pack_(Futaba/JR)