Recently a colleague in a doctors’ forum shared that he was still using a Treo 680! I thought that was amazing as my Treo 680 has long since died, though I still have a Palm Centro in good working order. Those were the good old PalmOS days.
Well being an old phone it’s no surprise that the battery life is not so good nowadays but you can easily get more juice with a new battery which is easily done as the Treo has a replaceable battery.
With my old Treo 680 and indeed my webOS phones, I have used the third party Mugen extended batteries without issues. I searched their website and they still seem to stock the standard size (no need replacement battery door) 1380 mAH extended battery . If you want higher capacity batteries, then you have to get the ones with a special battery door.
Mugen sells batteries for other phone models and from my past past experience with them they are pretty decently priced considering they offer free shipping and fast delivery from Hong Kong.
You could also try your luck in Ebay as a search shows several Treo batteries for sale but ymmv in cost and shipping.