Don’t buy anything. Tissue paper or toilet paper comes in handy for improving your grip for manual stretches. I’ve used vasoline and olive oil for jelqing. Olive oil is really nice. It slides easily and smells good.
Make sure you monitor your skin closely as you start your routine. Stretching especially places a lot of stress on the skin. I recommend gripping directly under your glans. That stretches the least skin and also works best.
Make sure you slowly increase the time and intensity of your workouts. Start with the newbie routine. Increase time and intensity every week or two. Your penis will probably feel different after a couple of weeks, if not sooner. It will probably start to look different after a month or so.
Enter your measurements in the PE Database.