Translucent AAA to AA Type Battery Converter Cases: converts 10440 (AAA) to 14500 (AA) batteries, 2 pack: $1.95
Do not purchase Li-Ion cells if you do not have the ability to monitor their pre-charge and post-charge voltage on a regular basis. That means having one of the battery testers like the one we sell on this site, or your own DMM (digital multi meter). This tester was made to test cells only up to AA in length, however by using an alligator clip (included) attached to one of the 9v test points, you can test 18650 and 26650 cells as well. $9.95
Spark SX5 Shim, to allow use of 18650 cells in headband battery pack: $4.95
The SX5 headband battery tube will hold a 26650 cell, which is a 'fat' 18650 cell. You could shim your SX5 battery tube to hold 18650 cells with a custom cut piece of PVC pipe from Home Depot for a buck. That's what I had planned to do. However, as the pipe might rattle, would not leave any air space for cooling the battery, and would weigh twice as much, I decided to travel the world on a trip that would justify a reasonable tax write off, and look for the perfect shim. Finally I rented a bike near Gweilin, rode south, and stumbled across a foward looking 18650 shim factory that built a shim with O rings, so I bought 3 of these and after eating Bamboo Rat in a Hot Pot, came home to sell shims and double my investment.