Can I use a pizza stone or a tile for the shelf?
Yes. but, you need to be consider how flat the stones are.
Choose the flattest, smoothest stones you can find. Take a ruler or other straight edge with you to select the flattest. Hold the straight edge vertically, and look for light coming from between the edge and the surface of the stone. Choose the ones with the least light showing.
Determining how flat the stones are
You can make the stones very flat and smooth when you get them to your studio. Put the surfaces together face to face and move one against the other in a circular motion. After minute or so of grinding, lift and take note of the areas which are showing the effects of the grinding. Where the stone has not been affected, are the low spots. The number and depth of the low spots will determine whether you wish to continue to even out the variations in the surfaces.
You can speed the grinding by putting a slurry of grit between the two surfaces. You can use a coarse grit of 100 or less in the grinding. Place a small pile of the grit and make a depression in which to put the water. Mix into a runny paste. And place the other stone on top and begin to move the upper stone in multiple directions.
Keeping the grinding surfaces damp will prevent any dust from the grinding getting into the air. You will hear a difference in sound when the slurry begins to dry out. This is the time to add a spritz of water to the grinding materials. As you check from time to time, you will see the areas that already are ground and those that are not yet evened. The grit will remain in the depressions and be clear from the higher areas. Push the grit onto those clear areas to continue the smoothing and flattening process. Continue until the surfaces of both stones are smooth and flat. This probably will not take much more than a quarter of an hour.
It is advisable when smoothing ceramic or glass materials to wear a dust mask. The dust from both are irritants, although not carcinogenetic.
When the stones are smooth, they need to be carefully dried. If you have the time, you can leave them to air dry for a few days. Even then you need to fire them to just below the boiling point of water and soak there for several hours. Keep the vents open, or the door/lid propped open slightly.
It will continue to be important to fire up slowly to keep the stone from breaking from thermal shock. The most rapid expansion of the ceramic is in the 200⁰C to 250⁰C range. This means that the rate of advance of firings should be slow until 250⁰C has been passed, no matter what the glass might survive.