Curiously, I wasn't aware that they came not recharchable in that form factor. Most of the ones I've seen are Lithium Ion.

There are some high powered, compact, and lightweight 18650 flashlights out there which make excellent bike lights. Some key things to be aware of however:

• You want to get protected 18650s, not unprotected. Protected cells have a built in circuit to protect against under or overvoltage conditions, which can damage the battery or even cause a thermal runaway condition. Protected cells are much safer.
  
• Stay away from the cheap Chinese knockoff 18650s which advertise impossible capacities. There's no such thing as a 4000 mAH 18650, but you'll find some batteries advertised that way, which often barely provide 10% of their rated capacity. Stick to the better brands.

These flashlights can make wonderful bike lights, and can serve as good flashlights for camping or other purposes when off the bike.