They drop when they feel like it. The monster type that drops them should be obvious when you see them.
Item drops in general aren't as frequent as they may have been in the old days. I haven't noticed _that_ much difference, but sometimes it seems like you're waiting forever for something in particular to pop up. I spent a day or two trying to get an Acoustic Guitarrr last week, for instance.
There are a few things that can help with that. Accordian Thieves can cast the Fat Leon's Phat Loot Lyric buff on you, which boosts your chances of drops. Disco Bandits can buy a skill that increases drops passively. The Baby Gravy Fairy familiar boosts your drop percentage, more so as it grows in weight. Some special items that require donations or megabucks (Mr. Accessory Jr., some of the esoteric familiars) can also help in that respect.
Likewise, Polka of Plenty, a Leprechaun familiar, and some other things add to the meat monsters drop.
I don't think there is a quest in the area where Disembodied Brains drop, so maybe you know and maybe you don't.
