Where is the best place to buy parts for my 1995 Ducati 750ss, thank you. 
Depends on what you are after? Anywhere from a dealership to eBay!! My suggestion is get acquainted with Google and the Internet and start building your database of suppliers for parts.
Ducati dealers should be able to supply most of the bits.
… and you could always advertise in ‘Desmo’ for any parts 
(Ads are free in ‘Panigale parts’ for DOC GB members.)