A good behaviourist always works on veterinary referral. This is because health and behaviour are very closely linked.
Your vet will give your pet a thorough check up to make sure there are no health issues which might be underlying his or her behaviour.
In some cases it may be necessary for your behaviourist to work with your vet in the treatment of behavioural problems, for example if the problem diagnosed is likely to benefit from treatment with drugs.
If a behaviourist does not work on referral from your vet, beware! They may recommend modification plans which are ineffective or distressing for your pet should they have any underlying medical issues.