Usually purpose made I think. I've never seen any for sale. All mine were turned up on a lathe but not by me.
I got these( BLACK RUBBER FERRULES SMALL-LARGE Chair/Table Furniture Leg Feet Cap Grip Ends) of ebay after reading a thread on here about do it yourself frame plugs. Might do the trick.
Jhp do them, I have them on all my bikes, as well as being crash bungs they double up as quick release axle bungs,front one has a hole in the one bung so you can put a bar through the hole and pull the axle out while the bung the other side can be hit with a dead hammer. Also available for the rear
