You can buy bored Surefire Z2 bodies (or send yours in for modification) from places like Oveready.com or from modders on Candlepowerforums. But upgrading to single cell 3.7V Li-Ion is a bit of work. You need a good charger, quality batteries (AW, Redilast), and then decide on size and chemistry. 18650 refers to the battery measurements BTW, 18mm diameter, 65mm in length, and 0 means its a cylindrical cell. AA is 14500, CR123 is 16340, etc. If you wanna make a quick leap to rechargeables my advice would be to go with LiFePO4 RCR123 batteries. Its a much safer chemistry, they're 3V, which means you use them just like you're using the CR123 primaries you've got now. I've been using Tenergy LiFePO4 batteries and charger from Batteryjunction for awhile now and they're great. The runtime is a bit less but they charge quickly and are very durable. If you're interested in doing some reading at the battery section of Candlepowerforums you can make a more informed decision on what you want to do.