TrackR pixel and TrackR wallet use CR2016 batteries, and TrackR bravo uses one CR1620 battery.
Battery life depends on usage, with an average maximum of 1 year.
These can all be replaced, and are available at the local supermarket or you can order a free replacement battery directly from TrackR app. Once you battery is low, just click the low battery indicator to order a replacement. The first battery is free! (+$1.99 shipping for customers outside of the US).
If you interested in purchasing extra batteries to have on hand, you can order batteries here.