I heard that Isocort is no longer available due to the FDA. It makes sense, because Isocort was essentially the same thing as hydrocortisone. I'm surprised that they were able to sell it for all this time without any trouble.
The only real substitute for Isocort would be hydrocortisone, since you can take precise doses. There are numerous adrenal cortex glandulars available, but none of them are standardized the way Isocort was. So you never really know how much cortisol will be in each capsule. It might be worth trying though, since there are people who do well with the non-Isocort glandulars.