...None. Sorry. The cheapest of cheap chillers out there will run you around $300 and they just aren't made for small tanks. They take up a ton of space and heat the air around the tank, so your room ends up hot unless you vent it. Just not practical for most tanks.
There are other things you could do to keep temps down. First off, having your temps in the low 80's, consistently, is okay. Temperature swings over a 24 hour period are more dangerous than if your tank always stays, for example, between 80 and 84. My tank runs between 78 and 84 year round.
You can buy a small clip on fan from Walmart and have it blow across the surface of the water to keep temps lower. I have two $7 clip on fans from Walmart on my tank, and just the two of them manage to drop my temp by up to 10 degrees on a daily basis (I don't even own heaters, but from my lights alone, my tank has heated up to 96 degrees before, and I will not buy a chiller for it).
If your tank is heating up, remove the heater. Even if you have the heater set to 76, or whatever. Their internal thermostats are not very reliable and taking it off completely can help.
You can also reduce your photoperiod if your lights are contributing to the heat.