Try your local hobby shop first, if they don't have what you need Tower Hobbies should.
I bet this is the one for your transmitter
http://www2.towerhobbies.com/cgi-bin...&I=LXH347&P=ML
Check your model number. (this one is 600 mah, you can get higher. I use 2100 mah NiMH in mine)
This will fit your reciever (again you can get higher capacity if you want)
http://www2.towerhobbies.com/cgi-bin...&I=LXH333&P=ML