Postum was a popular substitute for coffee until they stopped making it. I found a recipe for it here http://coffeeteablog.com/homemade-postum-recipes-recipes-requiring-postum/ But I havent tried it so I dont know how well it mimics the real thing. Ive also heard that some people will mix chickory with the coffee grounds to stretch them further.
If the doctor doesn't want you drinking coffee because it isn't good for you, is it because of the caffeine? Because if so, you'll need to avoid anything else that has it, and that includes most teas as well.
At one point I wasn't supposed to drink even decaffeinated beverages because they aren't truly "caffeine free". They still have small amounts of caffeine and due to a health problem I was supposed to avoid all caffeine. I don't think there are any coffees that don't have traces of it (unless the decaffeinating procedure has changed, which it might have), though there are definitely herbal teas that don't have any. For most people, decaf is fine because the traces of caffeine aren't an issue. If caffeine is the problem for the OP, she should ask her doctor if decaf is okay for her.