I recently found a split carb boot on the left side when I was changing jets. It was torn half way around under the clamp. Naturally, none of the local dealers stocked one. I used Permatex Gray gasket sealer. To date, I have clocked 6 hours on the bike and it is still holding and have not been left stranded. New boot due in this week.

 

I also ripped a carb boot on my old raptor. This was about an hour before a bunch of us were supposed to go riding, so I did what needed to be done. I improvised. The boot had ripped almost half way around, right in the middle of the groove for the clamp. I lightly sanded the inside of the rip, and cleaned it really well with brake cleaner. Then I put a few small drops of thin CA glue (airplane glue) in the rip, and held it together. It set in about 2 minutes. Then I spread some more CA around the outside of the tear. Finally I wrapped the whole thing very tightly in electrical tape.
I rode with this boot for almost 3 weeks while I waited for the replacement. A few times, the carb popped out, because I was afraid to crank the clamp down. But when I took the boot off, it completly held. I still have it, as one of thoes just in case parts!