Good question Greg. I'm making one that uses a 14Ah gel cell for my Polarie, but there must be commercial units out there.
Here's plenty of the 5Ah units for less than $25 and they're in Oz. I reckon that 5Ah is probably plenty for a longish evening or more. Then again, I'd rather the 14.... Nothing like running out near the end of a ripper run.
http://www.ebay.com.au/itm/5000mAh-P...#ht_1787wt_923
Warm regards
Chris Malikoff