ghost mushroomsFollow

You can buy them from Lhara at the Darkmoon Faire (limited supply).

They grow also in Razorfen Downs, Dire Maul and some caves in Zangarmarsh.

Some mobs in Felwood, Tanaris and Un'goro Crater drop them very occasionally.

I know them pretty well because I used to farm them a TON pre-BC on my warlock. There were 3 pre-BC places to find them:

1.) Skulk Rock in hinterlands
2.) The cave at the top of Jintha'alor
3.) Mara, just outside the zone

With the release of BC there are 2 additional places to find them

1.) Zangarmarsh in the cave in the south south east part of the zone, where there are bog lords
2.) The Northwest part of Zangarmarsh in the caves with the ogres

Now as I said I used to farm it a lot, and if you go and sit in skulk rock 3 will spawn roughly every 15 minutes. My warlock used to spend hours at a time in that little cave lol.