Made my Roman temple out of 5 inch (12 cm) wedding cake columns, which I purchased at the baking section of Michaels crafts here in the States. They look great painted faux marble. It is just a partial temple, to stand on the edge of the table...

Pet shop stuff looks good, but is usually designed to look like undersea Atlantis ruins, which may or may not be appropriate.
They wouldn't be too difficult to scratchbuild. Like Roman buildings, a Greek house is a basic box, perhaps with a tile roof, which can be represented by corrugated paper.