I don't know any retail stores that sell the 3.7v rechargeables you're probably looking for, so online is your best bet.
First, I'd need to know what size battery the
mod was designed for. Does an AA battery fit in there? If so, you're looking for 14500
batteries. If an AAA fits, then you'll want 10440s. If neither of those fit....yeesh, it's a pretty non-basic mod, then, and you should measure the area where the battery goes and tell us the dimensions in mm.
In all likelihood, though, it was built for 14500
batteries, and you can get them
here. Shipping takes a few weeks, though. You can also get them
here, and this one is based in the US, so shipping should be faster, although they are out of stock right now.
I'm sure other folks can chime in with domestic retailers for these batteries, but you can also just try searching on google. Just put "14500 protected rechargeable battery" in there, and try to find a local vendor with decent prices.