Thanks for the suggestions!
After I posted here, I had to go to Walmart, so I checked some out while I was there. Found these two, but I am just not "electrical" literate enough to tell if they are within specs to charge the unit in the on mode
GE 2-USB Car Charger - Walmart.com
GE 2-USB Wall Charger - Walmart.com
Those two have the current capacity ( at 2 amps) to both run and charge the HP-2 at the same time. However, the adapter must also be a 'smart adapter', that basically tells the scanner how much current it can provide. No way to know if that is the case with these two.