Newark has a bunch of choices that they can get from the UK warehouse with many under $5.00 a few under $2.00. Make sure you check the data sheet to verify they use set screws, the few I checked were all OK.
Unfortunately the URL from my search that was specific enough to get only set screw knobs does not work when used as a link so I made a less specific search with a link that you can modify to duplicate my search.
Start at the following link and narrow the search to these three suppliers, Mentor, Multicomp, and Multicomp-pro by control clicking their names.