Cabelas had their ice batteries on sale with charger for $20, they are backordered currently but a heck of a deal.
Otherwise, Harbor freight sells a small battery maintainer that has enough amperage to charge the ice batteries. Or you can use any small charger just unplug it when it is charged to prevent overcharging if it doesn't have auto shutoff. I use my boat charger on the 2 amp setting, about 3 hours if the batteries are fully spent.
Merry Christmas!
Spud one, spud two, if it went through, so will you!