I put DOT 4 in my kart, you can get it from any auto parts store or Walmart. MAKE SURE you do not mix any fluid together. I made that mistake and totally lost my brakes during practice.
Just be careful not to mix them, and do not get confuse DOT 5 with DOT 5.1. Different stuff. DOT 5 is silicone based, so it has a much higher boiling point and doesn't get contaminated like DOT 3 and 4.