Amazon has the Schumacher SpeedCharge Battery Maintainer and Charger (SEM-1562a) for $19 with free shipping on $25+. Automatically switches to trickle charging when battery is fully charged. Compatible with 6 or 12-volt batteries. Walmart also has it for $19 with free pickup.
I have a couple of these, they work fine.
Dumb name for a trickle charger