Unfortunately the only way to get them through Lowes or Home Depot is to order a pack of 5 or 6.
Local tile companies disappointed as well. The best I could find with was a 24x24 tile for $40.
Anybody knows of an online source that would be willing to sell and ship a single 18x18 tile? I tried and couldn't find any.
Edit: This is for the top of the table not under the Egg. I am usign fire bricks to support the egg.