I realize that KNO3 is virtually impossible to acquire in some countries but if you live in the US, making your own just doesn't make any economic sense.

The information is freely available on many web sites.  Save the $10 that some people want to charge for a tutorial. A Google search finds the links below. Once you see what's involved, you'll realize that setting up a chemistry lab and disposing of the waste simply isn't worth it for impure product. Check out these tutorials and see for yourself.

http://www.apcforum.net/forums/index.php?showtopic=732http://www.scienceforums.net/
http://www.geocities.com/CapeCanaveral/Campus/5361/kno3/ammnit.html
http://www.skepticfiles.org/new/027doc.htm
http://www.rotteneggs.com/r3/show/se/47404.html
http://www.musketeer.ch/blackpowder/saltpeter.html
http://www.apcforum.net/forums/index.php?showtopic=732