Yeah, a lot of the CIV artists all went in on a big order with a large printing company to get those done... a lot of those places are super expensive, so we had to get a group of like 20 or 30 people so it was affordable. In the past though, I'd just used Kinko's or any other local print shop.. Just set up the cards on regular size paper, get it printed color on one side (for the backs) and then cut them down myself.. Kinda of a chore, but it got the job done.
You can order generic sketch card blanks from bluelinepro.com too, but I don't think they let you create your own back image..