With cartomancy, the caster uses cards. For flavour's sake, they are probably larger than common modern playing cards, more like a large tarot deck. Their inherent magic is no greater than that of a spellbook, and in practice they function in much the same way. The mage may or may not have the ability to make cards themselves. Either way, to acquire a new spell, the mage must acquire a new card(s.) When casting the spell, the card is used in lieu of any material components, though it isn't 'used up.'
When the wizard has more time on his hands, he can use the cards for divining purposes. This should take about a turn (ten minutes) and should only provide answers to simple questions. One possibility would be using a short table for the result. I'll use the example of a mage doing a card reading for the question "is there a secret door in this room?"
- Yes or No - a clear answer, but any more specific than this shouldn't be possible, at least for a low level mage
- The answer is near, but you must do another reading - spend another turn laying cards and roll again
- The cards will not answer - for an unknown reason, the cards can never say if there is a secret door in this room or not
If the magic-user only has a handful of first level spells, you may want to say that each spell consists of multiple cards so that there are enough to do divinatory readings. Another option would be having a larger deck of cards, with only a few of which actually detail spells.
Other possible options might include limiting the number of cards one can have available at any one time, thereby limiting how many spells the cartomancer can have access to at once. This could easily be explained as conforming to any specific number of cards involved in a reading. For example, one of the standard layouts for tarot readings involves using 9 cards.