Got a few hints for ya, spend the first day after finishing the story to just find them all:
Skull Castle Courtyard:
Honestly, a pretty obvious hole I found it in.
Pirate Man:
Now where would a pirate have his treasure.
Pump Man:
It's real hard to see, what with all the water being pumped on your head.
Commando Man:
It's being quite a commando down there.
Sheep Man: (I stumbled across this one by accident, and wow.)
There's a small wall, in the normal wall, in the yellow wall. Wallception!
Wily Station Archives: (Second to last one I found)
One man's treasure is anothers... what?
Mars: (The final one that had eluded me.)
Finding this one felt like a tank rolled over me.
Punk: (I swear I saw this exact hiding "spot" for it before, but I can't put my finger on it.)
But it's location just broke me.