Out of the Abyss (5e)
The third adventure path for 5e, Out of the Abyss, features Underdark adventures. The second chapter of this book describes both prospects for foraging in the Underdark and seven separate edible fungi.
The Underdark (3e)
A little more about how the ecology of the Underdark works. Faezress--magic--provides the energy source for the growth of most fungi and lichens. Streams bring in nutrients, forming the other pillar of the food chain. Cave systems near streams and along gorges and ones featuring gigantic mushroom groves thus tend to be very fertile (by Underdark standards). This book also mentions small game in the upperdark (rats, lizards, &c.) and the fertility of lakes.
Drow of the Underdark... (2e)
...specifically mentions Drow cannibalism in cases of overpupulation/food stress. (And mentions insects, rothe, gardening fungi, &c.)
According to Bob Salvatore's various underdark novels, dark elves get by just fine on mostly domesticated meat breeds (like the Rothe mentioned by SPavel, each about twice the size of a yak), insects and reptiles, fish, fungi, and fermented spirits. (But these weren't designed as play-guides--just what we find mentioned in passing.)