Most Drymarchon will do best on a somewhat varied diet. They will eat rats, mice, fish, birds and other snakes (and pretty much anything else you feed them). I don't really like the idea of feeding mine snakes, because of the possibility of parasites. Some people do...not a problem really, just personal preference. Mine get rats (or mice for juveniles) and chicks (quail chicks for juveniles), and occasionally some fish. One note...they are very messy after eating fish or chicks (especially fish), but that's just part of keeping them.
Cribos are not really particularly rare. There are a lot of Black Tails available fairly reasonably. Unicolors and rubidus are harder to find and I don't think I've ever seen a caudomaculatus or margaritae for sale. Yellowtails are available, but fairly pricey. One more note here. Unless you are experienced at keeping Drymarchon, you will do much better to buy a captive born animal than wild-caught Cribo. As far as the Indigos, Texas and Eastern, you can ONLY buy captive born and with Easterns especially, there may be some Federal and State permit requirements. Wild caught animals tend to carry very large loads of parasites and acclimating them can sometimes be tricky.
As far as caging, they don't really need anything real elaborate, but they do need room and temperature and security options. Horizontal room is more important than vertical. They are big, fairly active snakes. If you do searches on this forum, and in the archives for this forum, just about every aspect of Husbandry for Drymarchon has been covered at one time or another by some of the best in the game. Dean Alessandrini, Doug T., Dan Felice, DryGuy, Carmichael, Regalringneck, Terry Vandeventer, Fred Albury and several others have had some pretty in-depth discussions on different aspects of Husbandry. Plus, if you have a specific question, chances are you can get it answered here.