These use a modern LiOn battery, which isn’t known to last long - the modern ones made within the past ~5 years seem to quit after 3-4 years, give or take. Depending on when you got it and how old it is, you could very well have an old battery since the used ones tend to come from former students who don’t want them after they graduate. Try changing the battery and see if you get any better results. Refer to this guide.
As far as finding one your best bet is to get it from TI here (Assumed US) or on Amazon under the part number 3.7L1200SPA. If you aren’t from the US, refer to this link to get it from TI.