Many wonder (I wondered myself) where to get coolant for the TT.
The TT only takes G12 which is a proprietary VW coolant. Therefore you won't find it in your auto parts stores, you have to go a VW or Audi dealer. It's about $25 for 1 gallon of concentrate, which gets cut 50/50 and makes two gallons, way more than enough to fill the system.
If you're replacing the coolant I HIGHLY suggest you do a flush. My heatercore had all kinds of gunked up coolant in it; after I cleaned it out, my car's heater warmed up amazingly quickly compared to before.