Is there a nice, if possible old but modernized, hotel near Milan, Italy (not in the city) that you could recommend for a 2-3 day seminar?
If you have no contacts there, you should get one. Hire a local fixer to do this for you.
Why don't you look at hotels on booking.com? If any of them seem promising, google them to find out more.
Look on Tripadvisor, or VRBO. But if you want rooms for 35 people, you'll need to book well in advance. Or call a travel agent.