It's the circular clip that is installed on the outside of the cap. When the cap boot is secured all the way onto the spark plug, the circ clip tends to lock the cap boot around the plug's hex area or just below it. That where the grip happens and makes it so hard to remove. The top section of the boot where the electrical contact is made is not the main problem.

On where to get the silicone, McMaster Carr carries it. Try their 51135K38 part. It's like 2$ a foot (and probably 4$ for shipping, oh well). Or just put "silicone rubber tubing" in your browser and check the internet. You want something with about 1/2" ID and about 1/16" to 3/32" wall. A short segment, say around 3/8" is stretched over the cap boot. Amazing how well it holds things. A foot of the tube will last you many many engines.