Create your own artist trading cards. You obviously can't trade something you don't have, after all. The only regulation is that they must be 2 1/2 inches by 3 1/2 inches in size.
Locate online swaps. With online swaps, you trade cards via the mail.
Locate an artist trading card swap in your area. See the Resources section below for assistance in finding a swap.
Swap your cards with artist friends. This is a great way to get started trading at first. If you don't have any artist friends, give them away to your non-painter friends.
Start your own swap if you can't find one in your area and don't want to trade online. Register it with one of the sites below, publicize it through sites such as Craigslist (http://www.craigslist.org), and put up fliers.