Not often given to advocating products, but moo.com are pretty excellent. Just got some mini business cards made by them. The are very economical, of good quality, and you can upload any image you want to form the back of the card.
I used a slight crop of my little drawing the nightwork, as one of the images. It came out looking small but lovely, and losing none its detail. Has caused some interest when I have handed it out.
Below: the nightwork.