For any of this to work, sometimes you have to tweak the lore. It may very well be "they can only eat sentient brains" is a myth, perpetrated by their own kind because they don't want to bother trying something else.
Mind flayers have several traits in common with the magog of the Andromeda series, and in the latter we see a very interesting story of one (Rev Bem) that turns against the brutality of their own and finds a way to live with the horrifying nature of need for wanton destruction and flesh-eating.
Also, this has actually been done in the Illithid adventures from 2E. Normally, ceremorphisis rebuilds the mind of an illithid, but sometimes they retain the mind of their former host and in the module's case, found a way to change their diet. You could stretch this to regular illithid and a cult or society among their kind that has come to see the normal illithid diet reprehensible and found an alternative.
With that in mind, there could be an established library run by illithid in a city (somewhere in Eberron comes to mind, maybe even Sharn...), who not only collect books, but read the mind of willing visitors and copy the information they learn down into written form. These illithid are monastic and grow an alien "wheat" they consume instead of normal food. They still reproduce by ceremorphisis, but do not do so against the initiate's will. In this way, the initiate's mind isn't consumed in the process, but instead merges into a new entity, aka as a sort of Trill, like Dax from Deep Space Nine - but with purple skin and tentacles.