While new cast iron can be found at most home and cooking supply stores, antique and other collectible cast iron can be a bit more tricky to buy. If you're looking for an older piece, here are some things to consider:
Examine the quality of the piece: the metal should be consistently thick, without any pitting or cracks (cracks are often hidden by grease).
Avoid cookware with swirls in the metal, as this can indicate poor casting. Properly cast iron should not have any hot spots and will resist scorching and burning.
Some dealers, such as Doris Mosier and David G. Smith, restore and season their pieces before selling. If you happen to find a piece — say at a garage sale — that is rusty but otherwise in good condition, you can restore it yourself, though it may take a little time. Both Smith and Mosier have detailed instructions on cleaning, restoring and seasoning cookware on their websites.
Here are some online resources to help you out:
Lodge Manufacturing Co.: http://www.lodgemfg.com
The Pan Man (David G. Smith): http://www.panman.com
Griswold Cookware (Doris and Bob Mosier): http://www.griswoldcookware.com
Wrinkled Willy Treasures: http://www.wrinkledwillytreasures.com
Iron Belly Antiques and Collectibles: http://www.ironbellyantiques.com
GCICA (Griswold & Cast Iron Cookware Assn.): gcica.org