Here is a tutorial on how to extract channels from normal maps. My previous tutorial was rather vague, so i hope this is better.
I am not sure whether there are other ways to do this, but this has been a technique that has
been working for me for awhile. Hope this helps!
You can save the .tga texture i have posted here to try it out if you don't have a normal map handy. Bear in mind that only .tga files have the right channels to extract.
.jpegs will flatten your channels out, leaving then unusable.
Download my sample normal map here