You might want to visit your local fish store and browse their decoration area. There might find some resin items by Penn Plax that simulate Greek like ruins, etc.

Also, they might have rocks for aquariums and if so; then you can determine if rocks can be carved or not.

Petsmart in USA has a line of aquarium decor called Top Fin and they also have resin "Greek" decor or similar: Top Fin® Greek Pavillion with Airstone - Decorations - Fish - PetSmart and Top Fin® Roman Ruins Building - Decorations - Fish - PetSmart. Not sure if there is that store at your location or if can order online; but hopefully this will help you a bit. Good luck !