RadioShack sells two power supplies that should meet your needs, but both are quite over your $25 gift card:
13.8v @ 15A (207 watt) for $80. This would be good for most chargers under 200w (Ice, Hyperion EOS 7i, etc).
13.8v @ 25A (345 watt) for $100. You could use two chargers at once or a single charger like the ThunderPower 1010.
A little on the pricey side, but the 25A is actually a better deal. It's switching too so it's efficient...