What I recommend is calling your local stores as Mr. Gaines recommended.
It is out of production, but I was able to find 4 tubes that way. It was on the top shelf in the back storeroom and they had to look for it. It helped I had pics of what the container looks like.
That worked for me, and I am sure there are lots of stores with old stock sitting on back shelves.
As for it going bad? I assume that if you have a sealed tube or partial used tube that is capped and inside, it should theoretically last a LONG time. Since it stays pliable and is subject to extreme weather after used, I would think it would not go bad stored inside on a shelf. Just my opinion though.
My only question is how many tubes does it take to do one car?
Thanks,
Dan in Kansas