i can't help you about tarot cards, but i can say that playing cards already existed many, many centuries ago, so they wouldn't be out of place at a renaissance festival. maybe instead of using the standard bicycle deck you could use some sort of vintage-looking deck like this one:
http://www.penguinmagic.com/specialorde ... hp?ID=9191
or whatever you like.
you could also do some sort of routine on the history of conjuring with playing cards: that would be nice and (as far as i know) original. and i think it would also be a lot of hard work on your side (if you do not want to make up the history)