Hi, no ticket office of SNCF (company that runs TGV) in Italy. Trenitalia as competitor dies not sell tickets for them. Maybe the SNCF customer service office at Milano Porta Garibaldi can help. 

You can also buy it at the SNCF Call Center or online at interrail.eu or eurail.com.

Here more about how to get reservations for TGV Milan-Turin-Paris