If weight is not an issue, craft stores with a decent floral section carry "rounds" of styrofoam in just about that size.
Also, the decorative baking section in the craft store will also carry cardboard rounds for cake decorating. Pair two with a long strip of carboard in the circumference of the rounds and voila!
Lastly, if you REALLY need weight, poured "concrete" stepping stones (in a perfectly round shape) are also available in kits in the mosaic / tile section of many craft stores.
--Melissa