Well i've been at this for about 6 hours straight, and I think I have a couple of good candidates. I'm dog tired so i'll have to wait until morning before I go at it again to make a selection.
I had just about decided to go with a BMS battery in spite of cautionary tales here, but then I noticed they don't include the battery charger, which was the deal breaker for me.
So it looks like it's down to these two 60v 20ah LiFePO4 batteries.

http://www.ebay.com/itm/60V-20Ah-LiF...item1c34a49313

http://www.ebay.com/itm/60V-20Ah-120...item1c3496f7e2