I did find flat caps online through a company that stocks weird fittings for making temporary sun shelters for cars and boats and whatnot... the flat cap is $1.06 each so you figure $4.24 for the four of them - but shipping is $8.45! How wild is that?? I don't think there is a place to get that kind of stuff locally.
I use pvc to raise my table, I used the rounded caps made to fit the pvc to protect my floors. You could use regular chair protectors to do the same thing, found at the local wallyworld. They are a hard rubber and shouldn't slide. No need to go to the extra time, effort, expense of internet shopping. :) Just a thought...