In the middle of the map, below the little shack that has the Nether Portal, there is a door with a treasure map on it, and marks all locations of where you can find the green headcrabs...

1# Can be found in the blue tower, near a staircase...
2# Can be found in the red tower, near entrance on a shelf...
3# Can be found somewhere in the ocean...
4# Can be found on a Ferris Wheel...
5# Can be found high up in the very middle...
6# Can be found in a bush, near the entrance of a room with a soldier statue...

Good luck and have fun hunting!

There is also now 26 new collectible trading cards of random TF2 players, these yield no reward, but may be fun to collect either way! I will not be providing hints for each of them though.