Welcome to the Benelli Forum.
There were 3200 750sei,s built in Italian factory ,and out of that 1058 were sold in USA.( sourced from info on Spanish Benelli Club ) With 900 sei,s the number is much less. There is one Benelli 900 sei for sale on ebay in Australia.
Despite from some opinion,parts( in my case for 750 sei )) are still available at least from two suppliers in Germany.
Some new parts are aftermarket due to classic nature of Benelli 750 and 900 sei . Used parts can be restored and sometimes it is better option.In my case I left original seat cover as I found out it is better still condition to new.