If you need that particular one, and you arent in a rush ebay is usually the best place to start. You could also post an ad on bike exchange https://bike-exchange.com/recent_adverts.htm
If you just need a tank and it doesnt have to be that one, the easiest and cheapest way is to get hold of an old fire extinguisher, drill and tap the bottom to fit the fuel tap and just modify the screw off top whatever way you see fit for filling. Dont forget a breather hole too.
GSX750 Ratfighter / SR125 Rat / GSX600F / Beamish250 / Honda70 Rat / Honda250 Rat Racer
Blackboard paint – Covers a multitude of sins!