Pull your battery out of it, and take it to an Interstate battery store. They should have one, or be able to get you a replacement. I've gotten them there for customers before.
I sell Automotive paint and body shop supplies for a living. I've found that if you charge them up before ever using them, and then only charge them when they actually need to be charged, that they last a very long time. I've sold different brands over the years, but I have one made my ATD that has been great for me for over 3 years. I have shops that have old ATD ones that are easily as old as mine or older, and I have shops that get new packs every 6 months. Don't charge them unless they need to be charged, and try to use them a little.