Some of the eBay sellers are great, Ping and Yesa are well proven. Some are scoundrels, so tread warily.
Personally I'd just email Li Ping for a price; he'll send you a PayPal invoice and the battery will arrive a week or two later. I bought a 36V 10Ah pack from Li Ping a while ago and it's very good indeed. It came complete with charger and BMS, all I needed to do was make a hard case to put it in.
Another option is to buy cells directly from one of the reasonably well-regarded Chinese companies. Headway will sell direct in small quantities, as will Thundersky and one or two others. The snag with this is that the cost of things like international bank transfers (and the associated risk) makes this a bit pricier than buying via eBay or using PayPal.
Nick (Tiberius) and I have both bought Headway 38120 cells; I have 64 of them for a 48V, 40Ah pack, Nick has a few less. They aren't that easy to assemble into packs though, I had to build a capacitor discharge resistance welding machine to assemble my pack.
Jeremy