Since the old method of making custom player heads from any site broke in Minecraft 1.8.4, I've made a small tool that helps making them in newer versions. You will still have to change your skin in the process, but once that's done the heads will always remain the way they were when you generated the command, regardless of skin changes.

Instructions:

1. Make sure you have a backup of your skin. If you don’t, get one by downloading your skin file here, for instance.
   
2. Go here.
   
3. Log in (upper right-hand corner).
   
4. Click the profile link in the top navigation bar.
   
5. Upload the skin file you want to make a custom player head from.
   
6. Go here.
   
7. Type your minecraft name into the box and hit the button.
   
8. Copy the command into a command block and activate it.
   

To get more heads, keep changing skins and hitting the button. Do note that due to a limitation by Mojang, you can only get one command like this per minute.

Don't forget to switch your skin back when you're done! :)