If money is no object, a respected vendor of Li-ion cells is AW (can be purchased in CPF Marketplace or from Lighthound.com). Personally, I'd probably buy something cheaper, but I'm not sure what is recommended. Check CPF.

I think using 18650 cells seems a lot more practical. DealExtreme SKU 20392 is generally considered to be a good low-cost 18650; a couple (much) pricier options would be AW and Redilast.