I combed almost every inch of Wildlife Exploitation Preserve and cleared every last pile of stalker goo I found and didn't see a single teddy bear. Where's the "stalker goo" that has the bears for this challenge?
Well it turns out they aren't in the "breakable" goo piles but actually suspended in the solid "drips" of stalker goo. You have to "collect" them with Pick Up/Action for them to count. This video has the locations:
Note this is another challenge you may as well do solo, only one person can grab any individual bear and they don't count for everyone.