Is there anywhere I can buy a real fender mustang neck, and would it fir on my War'Stang... AKA mustang built with warmoth parts.

Besides ebay please...
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nope, you cant find them, your only hope is ebay or to buy a new mustang and take the neck off of it.
Yeah you'd have to do serious looking if oyu don't take the warmoth or ebay route, and ebay isn't bad, there's tons of stores that sell fender parts fresh/new/unplayed.