Is this for an airworthy restoration or a static one? My understanding is a lot of P-63 parts are extremely rare and can only be obtained by trade and not bought.
I assume that you've contacted all of the surviving flyable P-63 owners/museums that own them?
In addition to these, you might want to contact Steve Hinton at Fighter Rebuilders, and Carl or Tony at Aero Trader, as they've both been known to have P-63 parts in the past. Whether they still have any or even have want you want, I don't know.
Also, try contacting the Hunt brothers in England, as they brought back 6 from Russia a few years back:
http://p63kingcobra.com/63_kingcobra_restoration.htmlI also seem to remember - maybe somebody can correct me on this - that Doug Champlain also recovered 4 to 6 P-63's out of Russia back in the 90's. I don't know where those are now or what the status of them is. You might try contacting GossHawk aviation in Arizona as Dave Goss worked for Champlain at one time and might have a lead on their current whereabouts or if they are available for purchase, trade, or barter.
http://www.gosshawkunlimited.com/There was also a P-63 project for sale on Courtesy's website recently that was a substantial collection of parts, though nearly complete, as I recall. It was in the Santa Rosa, CA region, if memory serves me correctly. I don't know if that project sold or if the owner decided not to sell. Perhaps somebody here can give an update on that project and its current disposition. The current owner of that project may be of use to you.
Finally, keep an eye on Ebay. You would be surprised what stuff pops up there from time to time. I have seen various P-63 items at auction over the years, so you may have to be patient to find what you need.