That's a custom made tank for that particular bike.
The only aftermarket busa tanks available are stock style aluminum, or one off customs.
The one pictured appears to be sheet metaled on to the stock tank(could be a fiberglass job too, I can't see enough detail to tell).
You can buy fairing kits on ebay that come with plastic tank covers.
Those tank covers are stock style also, but could be altered to accomplish the same thing.