Plastic shims will work. Cut them so they stay hidden under the toilet.

They are available at hardware stores, but I haven't seen them in HD or Lowe's.

Thank you both so much. We will get some today. Again, Thanks, Debbie

I use a piece of 1/2 ins.copper pipe 1/2 long flatten it put under toilette.seal it to hide it.If you seal all around the toilette leave about 1 ins at the back not seal,that way you will know when your toilette leaks.(in the future.)

i used a couple of roofing shingles for a shim on mine, but i only had to deal with 1/4" on one side.

I used the little clear plastic surface protectors that you use on lights, etc. to keep them from sliding. They have several benefits: they are self-stick, so stay in place on the toilet, and can be cut to fit the bottom rim of the toilet. In one case, the actual dot was too high, but the sticky plastic sheet that connects the various dots was just the right height (about 1/8') The link shows one kind, but you can buy them at any hardware store.

Here is a link that might be useful: Scratch Guard