I go along to the garden centres. They have all sorts of stuff that people use in the garden and rockeries, this is the sort of stuff you can also use in a fish tank. Pieces of rock, cobbles, and different size potts. I don't tend to buy any of this stuff from fish shops, if they do stock it, they charge you an absolute fortune for something that you can pick up for very little money.
I gave my rocks a good scrub with the brush and poured boiling hot water over them (bloody painful it was:lol: )
A lot of the clay pots aren't painted so just give them a rinse and they should be okay. Put it this way, I've never had a problem over the years.
I know it mean, tank does look nice with a rock or wood. I've got two huge pieces of wood in my new 300 gallon tank along with quite a bit of rock and some cobbles. I'll post pictures to