You need to do this in 1.7.2 or over. This method only works with internet. If your on a server, this method might not work depending on what server your on.

Doing it in inventory-

1st: Press T To Open Your Chat.

2nd: Type In this command-
/give (Your Username) minecraft:skull 1 3 {SkullOwner:(Anyone's Name)}
Example- /give SergeantGadren minecraft:skull 1 3 {SkullOwner:SergeantGadren}

How To Put It On A Command Block-

1st: To Get A Command Block, Use These Commands-
[1.8] /give (username) command_block
[1.7] /give (username) 137
Example: /give Pocholo2004 137
OR
/give Pocholo2004 command_block

2nd: Type In This Command-
/give @p skull 1 3 {SkullOwner:(username)}
Example- /give @p skull 1 3 {SkullOwner:Pocholo2004}

Databases where you could get Heads-

Player Head Database- http://minecraftplayerheadsdatabase.weebly.com/mini-blocks.html
Head Freshcoal- http://heads.freshcoal.com/maincollection.php
By goCreative Arcaniax- http://www.planetminecraft.com/blog/minecraft-playerheads-2579899/