I was trying to google a bit, but didnt find litterally anything about some races trading with Illithids ... wiki says that "beholders, drow, duergar, and other dominant races of the Underdark respects them" ... but that is all.
Yeah and they hate the sun. So maybe there is some coalition with the Sharrans. As for trade relations, I find those with the Drow profitable for both sides. The Drow give the Illithids slaves (brains), the Illithids give the Drow collected (magical) weapons that do not turn to dust for their surface raids. The Illithids do not need equipment, after all. Win-Win-Situation.
