Made them at home myself actually. Where to buy them I don't know, sorrry.
I was in my toolroom fixing something and I saw this 2" square piece of wood that fit perfectly into the 2" square hole in my weightbench, so I cut a board in half, cut circles in it and attached evrything.
I had all these materials for years down there, glad I finally figured it out.
Thank you for asking, sorry I couldn't be of more help