The batteries the mods use are very popular in flashlights so fairly easy to obtain. I wonder if a R/C hobby shop would sell them locally? I ordered mine from dealextreme as they were extremely cheap, but will take awhile to arrive from China. The black ones with the flames on the wrapper Trustfire 18650 2400mah. They have the protection circuit known as a PCB on the bottom. I wouldn't use batteries without this PCB. The website also sells the charger; I personally use the Trustfire TR-001 charger. The most highly regarded batteries are the AW brand. Which comes in at roughly 2x the cost of the Trustfire brand from china. But people say they really perform well. The lighthound website sells AW also with the PCB. AW has batteries which do not have a button top so I would probably choose a button top design over the flat tops. Not sure if the SB or Omega would function on flat tops.
I also picked up some plastic battery holders for them to keep them from just laying around. Come in colors so you can stay organized with charged/discharged batteries.