"There are no fewer than 5 different, incompatible "26 inch" sizes!"
I got into this variety of torture a couple years back when we bought a couple very cheap used bikes from a fellow the next town over. The tires were rotten and I had to replace them. These cheap bikes are really meant to be used... the bike shops all just laughed at me - nobody puts new tires on those bikes! One bike was a Magna brand - maybe both were, of slightly different vintages.
I went to
http://www.biketiresdirect.com/and looked up the little numbers on the sidewalls and I did manage to find the right tires. But it was pretty much a nightmare. The bikes were such junk they we went out and replaced them, with a Jamis and a Giant. So now I have a small stash of really odd sized tires!
About a year later the STI shifters on my Trek 520 failed and then I started around again trying to find compatible parts ... I could see myself replacing the whole drive train because all those parts are just unavailable... that whole slippery slope led me to a Thorn Nomad! Am I insane or is the world insane? At least I got a nice bike out of it!
Jim