You can make your own calibration solution by following the info in this article:
http://reefkeeping.com/issues/2004-06/rhf/index.htm
You really want to do it with a scale though and make sure you get the solution right. The AMI calibration solution is cheap enough that I wouldn't bother.
My refractometer was pretty good at first but it has been off more and more lately, probably dropped it or something. I was just lucky enough to have heard about the potential problems with the calibration already so I didn't go and screw anything up. If your livestock looks good make sure you can confirm your measurement before you change something.
*EDIT*
I don't like the coke bottle thing in the article and I had a few minutes to kill so I figured out how to make up a decent solution if you don't have a scale but do have a fairly precise liquid volume measure. You need to mix 3tsp table salt in 491ml RO water. Assuming that the tsp of salt measures 6.2g, it should be pretty close, that will give you a solution that should read 35 ppt or 1.265ish.
Turns out the calibration I did to my RHS10-ATC based on a Deep Six hydrometer reading was bang on.
Back to the drawer with the refractometer.