Just take your belt to the auto parts store and have them measure it. I just got a belt for my Craftsman Mower for $9.99 at O'Riley Auto Parts. It was $35 at Sears.
Chris
I have a MTD and also couldn't find the drive belt number listed in the catalogs. I did just what Diamondpilot suggest. I took my almost completely distroyed belt to a Do It Best hardware shop (like an Ace hardware) and they were able to measure it on a little belt measureing device. The new one fit just right. It was about $10 too!
jb