Toilet bowl shims
 

I have a toilet flange (in concrete slab) that is set too high and the bowl rocks. I have used the plastic shims to level the bowl and it works for a while, but over time the shims move and the bowl rocks again. An old timer I know says he used to buy a toilet gasket (rubber or neoprene) that was bigger thyan the toilet base, had a precut hole for the flange and was 1/4 to 3/8 inch thick. He would set the toilet on the gasket, bolt it down and trim arounfdthe bowl base with a razor knife.

Anyone ever hear of this? Are they still made? Where can I buy one?

Cut your own from rubber stock available at home centers. Or, add a touch of Goop silicone sealant to both sides of the shims, then bolt down the wc bowl. That should glue the shim in place so that id doesn't creep due to flexing.

I just use plaster, if it's real bad I use lead and plaster.
Set the bowl down to see where it's going to be
Trace around the outside of it with a pencil or something and pick the bowl back up
Mix up some plaster of paris and have a bucket of water and a sponge ready
Put down a nice bead of plaster around your line
Set your bowl in place and if you need to use some lead shims put them in
Bolt the bowl down
Clean up the excess plaster