If you have constraint on budget, 2nd hand chiller still can be costly.
The cheapest you can get may not work well or consume more power.
Anyway, you can always browse those local aquarium forums for 2nd hand offering.
Or you can post up a message stated wanna buy.


Just an idea:
In another way, if you (or someone, like your friend) good in electrical, you can try workout a thermo-control fan.
You need a electrical controller that can sense the water temperature and trigger on / off of few fans.
The thermo-sensor may be expensive too.

HTH