I'd look around or ask around your area for an Advanced Obedience class. The class I'm has helped me a lot prepare for Regis' CGC/TDI test(they are quite similar). They do the CGC tests there, and every once in a while half of the hour period is dedicated for prepping for a CGC or TDI test.
At the local hospital, they train and test for TDI. I'd check that also. It would be GOOD to go to a classes if you can find them!
Good luck I'm excited for Regis to take his. His CGC is at the end of the month and his TDI is in october!