I suspect this is a regulatory issue. It tends to be small, specialist companies, sometimes hotels or even local tourist offices that organise these tours. If they are in the EU and include flights they would need the equivalent of ATOL bonding which would be a bit much for them, especially if a large part of the trade is coming by train or car.

Can't really recommend any sites but what I will say is that Italy is a gorgeous destination and one that I would strongly recommend if you want to hop on your bike...especially in Summer. Although I would definitely say avoid the central summer period, when the weather is really hot, and winter too, for the problems associated with the bitter cold, snow and/or ice. So transitional periods are best. The Piedmont is an ideal destination for a bicycle tour...great food, wine and scenery...can't go wrong! Enjoy wherever you go...