http://www.battlepack.com/ULTRAPacks.asp
The IB 3600's are very good. The IB 4200's (WC) are more powerful and run long but are very finiky. I bought some packs off ebay. Off brand NiMH 3500's for about 14 per pack (6 cell and they get my SV to 25-27 mph. Good cheap fun. Somthing like these.....
http://search.ebay.com/battery-packs...saobfmtsZinsif
That's top and bottom of the barrel. Doen't sound like you are trying to break records.