Newbie - Gen 1 Aux Battery Problem

I am new to the forum and am not sure the appropriate sub-forum to post this question . Short story, my father in law bought my son a Gen 1 2001 Prius (I wasn't part of the purchase nor consulted. It was a private party sale. I don't want to get sidetracked on the wisdom on the purchase). We now need a new 12 v aux battery. We have had to jump the car several times. The battery will charge but not hold the charge. I had it tested and the battery tester recommended the battery be replaced. I called the Toyota dealership and they said to replace the batter was $420 with the battery cost at $350 or so.

I am pretty efficient with a wrench just not experienced with hybrids or with a Prius.

Where can I get 1) a reliable used battery or 2) a reasonably priced new battery?

Thanks,

Roger