Home depot should have it. It'll cost like $5-10, and is a pretty good trick for saving some money. Like Mike said, just cover up one side of the mount (I've heard cardboard works, but I guess foil would be fine too), squirt the sealant into all the voids of the mount and smooth over the open side of the mount with your finger so it doesn't look like glop. If your mounts aren't totally shot, you should be able to do it with them still on the car. Otherwise, just remove one mount at a time and spread them out over the course of a week.