K, been at this game and spending money as needed for 6+ months. I have one venom from somewhere long ago. I see an achievement for using them, but I swear they have not been available for at least 6 months or I'd have them.
I think at one point they were supposed to drop from maybe airstrip dungeon, but it was actually dropping different talismans as noted elsewhere in this forum.
I know they aren't that useful, but if you're going to have an achievement for them, at least offer them in an event or one of the shops.