You'll find it here:
http://www.coastairbrush.com/proddetail.asp?prod=Restorer
I keep mine in a pickle jar. It lasts forever, even when you can see old paint floating in it. I sometime backflush with the restorer, and shoot the excess right back into the jar, if I think the brush isn't shooting right.
Every few months I break down my air brush and soak it overnight in the jar. The next day I use the needle, blunt end first, to clean any old paint that might be still in the needle barrel, use my compressed air to blow through the brush to clear anything else that I can clear, then I reassemble the brush, putting a couple of drops of needle oil onto the needle before I reinstall it, and a drop in the bottom of the trigger before I reinstall the air hose fitting, so the oil blows up into the gun when I hit the air again.
Thinning your paint, and backflushing with clean water between colors, will go a long way towards keeping your brush functioning well.