I use the rechargeable NiMh batteries from my Canon ultrazoom camera in my girl's vibrator. They work fine.
If you're going to buy new batteries, I'd recommend the Eneloop low discharge ones. They hold their charge much longer than my standard NiMh batts. The Maha brand of chargers are really good too.
I like buying them from
http://www.thomas-distributing.com/index.htm