Gas Tank Parts Replacement

Hi, I have 2002 Nissen Xterra 6cyl., my 'service engine soon' light has
been went off for awhile and indicated 'a small leak'. After few reset, my
mechanic finally find the leak was at one of the filter or membrane
surround the fuel tank or one of the valve. He explained to me that the car
was slowly leaking vapor. Since my fuel tank was so rusted (from salt). If
he tried to change whatever it cause the problem, it might end-up changed
the entire tank, filter, etc, which could cost up to $1000+. He suggested me
to search for car part that from country that didn't snow, ex, Arizona,
California, found the used fuel tank parts to replace it.
However, when I searched, there were TOO MANY parts that were related
to fuel tank and vapor system.
1. Which parts do I need in order to replace the entire fuel tank?
2. Where is the best place to look for the parts that I need? Junk yard?