I think im going to be getting an 82 yamaha seca 400, but he says it needs a new gas tank and lights, possibly some other parts. does anyone know where i could find these parts, or if they make any aftermarket parts that would fit? dont really know much about bikes. any info/advice would be very helpful and appreciated. thanks.

Ebay and craigslist are good places for parts. Local salvage yards may have some items too. Things like lights, handlebars and grips can be bought at local cycle shops. Some hard parts (gaskets, o-rings, levers, etc) are still available from Yamaha.