a friend of mine last year started a nice quarterly about beverages. she worked with the major players to get advertising and kick-off funding. the issue was distribution.
here you have an advantage: us games, llew., etc. all would love to be your advertisers, and they can also help you with distribution in terms of places like borders and barnes & noble. they can also do cross-promotion with you.
it's certainly in their interest to have an independent magazine on the subject, because that helps to create a broader market and more consistent marketing. by working together with them but keeping good journalistic standards (particularly in reviewing decks) you not only create a nice thing for yourself, but a market space of deep value to readers and publishers.
your value proposition is that while the internet has a ton of tarot info, it is scattered. you have to go to a lot of websites, which takes time. and time is what many people don't have so much of! by unifying tarot information in an attractive, positive, and trustworthy place, you can offer something very helpful to the market.
i'd approach the card publishers right away. good luck!