I keep hearing about this special 21mm strut tool, where can I get it? How do you use it?

 

Here is just one place to get it. http://www.eurosportdesign.com/p-438...al-socket.aspx
I beleive many have changed the shocks without it though. Read these-
http://www.pelicanparts.com/techarti...eplacement.htm
http://www.pelicanparts.com/techarti...eplacement.htm

 

Another option would be to make the tool yourself for about 1/3 the price. Purchase a 21mm deep socket and cut a "window" with a Dremel.
As to how to use it the socket holds the nut. The "window" allows you to insert an allen wrench and hold the shock shaft while you tighten the nut.

 

Using a 21mm sparkplug socket with an allen tool stuck through the end worked much better for me.