I have been using butyl tape for years. You can buy rolls of it at most RV centers. I get it at one near my marina and pay about $5 a roll. That is why West Marine and others don't sell it. It is way too cheap.
Another benefit - the leftover 4200 and such in a tube will harden and get thrown away - you don't have any waste with butyl.
I keep a ball of it handy while I work on the boat. Need to plug some holes temporarily? Just stuff some butyl tape in the hole. I plugged some holes when I removed deck hardware when I was painting the boat. Had no leaks at all.
I also use a bit of it on the end of a screw to "glue" it to a screwdriver when needed.