I have lot of 18650 batteries, so I ordered this from ebay www.goo.gl/HeAJ1 (btw. shipped the same day I ordered it). Maybe it will work There are different versions, for two 18650 cells (max. 1A output), for four cells with different max. outputs from 1 to 2,5A (2,5A version is probably the same as 2A version )
I think it is better solution than using AA and similar batteries (one 18650 battery has more energy than three AA batteries...), or \"fixed\" battery pack where it is impossible to change cells.
18650 are really cheap (about 6$ for \"2600mAh\" (actual capacity is about 2300) battery with sanyo cell). It is possible to get them almost for free from used notebook battery packs, usually if they are not too old they can have inside cells that have capacity about 1500-2000mAh (some can be bad, recently I opened one year old HP battery pack and all cells are good, but in notebook protection tripped after 15 minutes ).
They hold capacity very long - if they are made from quality cells, like panasonic, sanyo, samsung, lg... Some are even after 2 years in very good condition, interesting test is here http://budgetlightforum.com/node/4028
Sorry for my bad English, I hope it is understandable