That was fun. Smooth-On makes good products. He orders from a third party on Amazon. You can order directly from the Smooth-On website or they're distributor Reynold's Materials.
Mold Star is a platinum based silicone. It has a long (approx 20 yrs) library life so your molds will last a very long time. However, it's very susceptible to contaminants. Make sure everything is clean or you may end up with a puddle of sticky goo.
If you decide to go with another silicone make sure it's pour-able straight out of the container. Most silicones are very thick and will require a vacuum chamber to get out all the air bubbles.
If you want colored, clear, or clear tinted knobs you'll need another resin (obviously) which Smooth-On also has and they have tints and colorants.
If you decide to make the entire knob (not just the top with flat bottom on it) that will require a 2 part mold (at least) so don't forget mold release and keys in the mold so both pieces will line up. A pour spout and/or relief holes aren't bad idea either.
He uses a mold release. You don't really need one with silicone most of the time but it doesn't hurt. But, silicone will stick to other silicone - see above comment.