Both my Ace Hardware and Hobby Lobby have a small selection of wooden knobs but, I don't know if they have a match for the one you pictured.
I have made my own knobs from time to time but, I don't have a lathe so I use the scroll saw (cut them like a 3-D pattern) and end up with square knobs or the drill press and a wood rasp. Needless to say, you have to start with a larger than necessary piece of wood and work it down until it is what you want and cut to length.
Rog