The spec. for the light indicates that the battery pack is a 6x18650, and these are normally arranged as three sets in parallel of pairs of serial 18650 lithium ion cells. Each cell is nominally a bit over 4V, so the voltage of the pack is 8.4V and with three times the Amp-hr rating of an individual cell. The battery pack listed above, with 8 cells, will also work fine and gives a higher Amp-hr rating for a longer tun time.

18650 cells are very common (the number indicates a diameter of 18mm and length of 65mm) and can either be bought individually or in a variety of bundled battery packs.