I got mine for 50 cents a piece at walmart maybe? They are 4 inches and I used them for the convicts, bolivian rams, and was going to use them with the honduran red points.
Ceramic is aquarium safe, when I was in a ceramics class I made a ton of breeding/hideouts for my fish.
I have had them in my tanks for 6 months now and no ill side effects. I am pretty sure every ceramic piece is going to have some hard minerals in it, like trace elements and such because they are just made from dirt. But I havn't had any pH Spikes or anything like that in the tanks I have used them in. Just go to the garden section of any store and see if they have some.