https://www.ebay.com/itm/COMBO-OF-2-...0AAOSwsc5e48Qg
Here's a pair of 6V 12AH batteries, buy these and put them in parallel and you've got 6V at 24AH available to your radio for under $30 shipped.
Keeping them charged is then a simple matter of any of the multitude of cheap chargers also sold on the 'Bay ........