Which batteries did the helicopter use before? Why not start there?

Unless you are absolutely sure of what you are doing, I would strongly advise against just selecting lipos as a power option just because. Google "Lipo fires" for an eye opener.

Ok, here is what I found:
The original helicopter flew with an 8.4v nimh battery and people suggest an upgrade to 7.4v 800mah lipo.

Here is a Google link to cheap lipo chargers

Good luck, and be careful with those lipo batteries.

Rafael