I'm doing the same thing--kudos for being willing to pay for the art, rather than trying to get artists to do it for free.
The way I went about it was going onto DeviantArt's forums and posting in the "paid work" folder what I was looking for (including rough details of the art style), and how much I was willing to pay per card. You should decide if you need to own the art (in case you want to sell prints or t-shirts, or the deck itself to a publisher), or if you're willing to let the artist keep ownership, and have a contract for either scenario. Myself, I own the art, but I allow the artist to also sell prints in a limited capacity. Then look through the responses you get, and find an artist (or multiple artists) who have the skill and style your deck needs.
I did it this way even though it means the deck is taking awhile to construct (as I save up money for each card)--I could have tried to find an artist willing to do the work for a split of future profits, but I, personally, don't like that arrangement, as it puts the burden of risk on the artist (and I have too many artist friends to do that
).
As for how much per card, that's up to your budget, and how detailed your preferred style is. If you want Vallejo-style paintings that take hours or days, but are offering $50 a card, you're probably not going to have much luck.
And also how established your artist is--someone just starting out will be more willing to work for a lower price than someone who's already getting steady illustration work.