The Melbourne Aerobatic Club held a
workshop here last week on this very subject. Advice was similar to that provided by shortstripper and AerobaticFlyer.
What's your objective? Sunday afternoon loops or something more serious (but be warned - aerobatics is addictive).
Do you want to stick to tricycle gear aeroplanes or not?
There are instructors who'll teach basic aerobatics in a Pitts and others have commented on the pros and cons of that. The Cessna Aerobat is a great little aerobatic trainer. The Decathlon is an excellent aerobatic trainer.
The above considerations should result in a shortlist of flying schools. The final step is to choose a good instructor and some-one that you'll get on with.