Crack that case open if they won't help you. I bet you'll find a sealed lead acid battery in there. Alas SLAs require intelligent chargers to last any length of time. Most manufacturers that use those batteries just straight wire a power supply to the battery, which cooks away the electrolyte in no time.
If I'm right, that's what will be in there. You can go to battery wholesale dot com and match specs and terminals. SLAs (at least not smallish ones like this would have) are not too expensive.