What do you mean get a certain player's head? If you're talking about the skin, well then, it's a lot harder to. Even I don't know how to.
For just the face for non-slim skins (ik, I'm outdated with these stuff, you no longer say slim in 0.13), I have dumped the offsets of the pixels of the face. https://github.com/LegionPE/LegionPE-Theta-Base/blob/master/resources/head.json Note that the offsets have to be divided by half to indicate the byte offsets. Each number represents four bytes as one pixel.
I suggest you to create a skin totally in #AAAAAA but the parts you want #BBBBBB. Then dump the sent skin and locate the four-byte pixels of 0xBBBBBB??, using a hex editor.
Simple answer is, you can't. Pocket Edition doesn't have the ability to set the texture of a player head to that of a skin. The only way I see this being possible is either when they implement it or implement the ability to send texture packs through servers.
So you want to create a head item with player's skin? I think it isn't possible, the only possible way is creating a cube/slime render, giving it somehow the head part of the player skin.
Did you know that they can take off a skin's head? The face will be taken off. Not a matter of where to fetch it, but how to process it.
