I guess any sheet would be okay as long as you didn't get any soap on it. I wish there was some sort of ovenproof paper to set under the mold before you put it on the sheet. With my gloved hands sometimes I don't know where I've touched and am used to being able to just set the mold on some freezer paper.
Thank you for the great ideas. I actually just got back from the store and came home empty handed for mulling over whether to buy a Pyrex baking dish or not. But they're heavy and cost $16.99. These other options would be much cheaper. By tomorrow night I should be gellin'.
I bought pan lining paper at Walmart. It was only $2.98 and I think if I use a regular cookie sheet I'll just put a couple of layers of the paper down before I set my silicone soap mold on it to gell.