There are many ruins of roman amphitheatres around Mediterranean sea.
You can find 62 of them here (all in high resolution). At least 60 more are in low res areas.
Caution : there are only Amphitheatres here (full elliptical form) and not Theatres (half-circular form).
For ancient theatre, you can see this post :
http://bbs.keyhole.com/ubb/showthreaded.php/Cat/0/Number/145308/an/0/page/0#145308
Do you know others ?

thanks to :
http://italia.novaroma.org/signaromanorum/sr_lista_anf.htm
http://perso.orange.fr/claude.philip/Pays/italie/frame.htm