You can look for a mentor in any reputable tarot organization; in the US, it is the ATA (American Tarot Association) in the UK (or anywhere else), it is TABI (Tarot Association of the British Isles).
But, know that any other person, however experienced, has their own way of looking at the world, and consequentially how they read the cards.
Personally speaking, your best mentors are the people you read for. They are the ones who give you practice, challenge your assumptions and question your interpretations. Again, you can do free readings at either of the two organizations mentioned above.