there are tons of great options for using lithium-ions in surefire lights. But if you haven't bought your light yet, I highly recommend "building" a SF light up with a leefbody so you can use 18mm diameter li-ion cells, this will give you about 50% more runtime and slightly better brightness.
A leef 2x18500 or 2x18650 body is a great platform to start your builds on. I guess they are in limited supply so you might have to wait a bit to get one. But they are sold at lighthound.com .... also keep an eye over in BST. A C tail C head body, combined with an appropriate SF bezel and tailcap will allow you to run "D26" style lamps from many manufactures, including SFs own amazing wall-of light P91, or more conservative P90. LumensFactory is offering some great in-between options, A P90 falls in at about 1.2A, P91 about 2.6A, LF has offerings at 0.8A, 1.2A, 1.5A, 2.0A, so you can really fine tune a balance between runtime and output to meet the need.
I don't generally suggest a 2xCR123 size light in incandescent anymore unless you just really favor the incandescent output, cree LEDs in this form factor are blowing away incandescents in output and runtime.
You can simply use 17mm cells in a SF body, which simplifies things as far as putting the light together goes. Click on the link in my sig line (same link stockae92 just replied with) and scroll down to the compatability chart to see what can be done... Please read through the guide, understand the risks vs benefits. Buy quality protected cells and quality charger only. Seriously consider spending the bucks on a Pila IBC charger, it will eliminate 95% of the headaches involved with properly using li-ion cells.