I, too, have this model phone and love it. The headset battery barely lasted 9 months. I've been bumped around to many places including Thompson, Radio Shack, etc. I do believe Thompson (800-338-0376) has the battery for the HANDset, but they DO NOT have the battery for the headset. The HEADset battery is available through partstore.com (on the internet) (866-925-7278). This shipping document gives you the pricing info:
You may well find that the HANDset battery is available there, also. The RCA HANDset battery part number is 5-2683. You will need this to cross reference if you use partstore.com.
Warning: Radio Shack will say that they have the replacement battery. I had to send the one they ordered for me back to Radio Shack - - don't need to say more!!!
Hope this helps.