"King Philip's Head" seems to be a long term research project. The entry for "King Philip" in Encyclopedia Britannica ends: "On Aug. 12, 1676, a Wampanoag informer named Alderman killed King Philip. Philip was beheaded and quartered, and his head was displayed on a pole in Plymouth for more than 20 years."
Searches in JSTOR indicate that a lot of information is lost. "Search title only": ‘king philips war’ gets 7 hits, including:
Restraining Atrocity: The Conduct of King Philip's War.
James Drake. The New England Quarterly, Vol. 70, No. 1. (Mar.,1997), pp. 33-56.(P.40: "during a chase, an Indian fighting with [Benjamin]Church shot and killed Philip"; "Church...had the traitorous corpse beheaded and quartered."... "In a final act of vengeance that went beyond law, Church ordered that...not one of his bones should be buried.")
JSTOR "search text": ‘king philips war’ and ‘philips head’ gets 1 hit:
Joining Signs with Words: Missionaries, Metaphors, and the
Massachusett Language. Laura J. Murray. The New England
Quarterly, Vol. 74, No. 1. (Mar., 2001), pp. 62-93.
(p. 65: "Philip’s head was impaled and displayed in Plymouth
from the end of the war in 1676 (when [Cotton] Mather was only 13 years old) and for many years afterwards, so it is possible that Mather removed the jawbone once the flesh had rotted away from the skull". Also, footnote 7 refers to the book "Red King’s Rebellion: Racial Politics in New England 1675-1678", NY Atheneum 1990, p.201).
So JSTOR seems best for some earlier full-text online accounts, if you are near a large academic library that has access to it (or if you have a student/staff ID giving you remote access). It is also well worth looking in books under the subject: King Philip's War, 1675-1676. You probably would need to look in a fairly sizable public library - unless you are in the Massachussetts area - especially Plymouth. The University of Maryland Libraries have about 52 books under this subject, and I would expect New England universities and colleges to have far more.
The database "America History and Life"(available at large academic libraries) gets 96 hits for the subject "King Philip's War". If you need less scholarly articles - the database Masterfile Premier (Ebsco) is likely to be available at nearby public libraries. The search: king philip's war (no need to use double quotes in Ebsco) gets 67 hits - 34 of them full-text online. Looking at a few of them, I find nothng more definite than the "more than 20 years" quote from Britannica.
Given the importance of this event, it is even conceivable that present day detective work might come into play at some point (if it has not already) - with search for traces of remains, etc. There is plenty of research being done; Digital Disertations database (again, at large academic or research libraries) gets 28 hits on "King Philip's War" - 17 of these PhD dissertations since 1993. It would be a tough task, but borrowing some of those theses on Interlibrary Loan might prove useful because of their huge bibliographies. Looking in books under subject "King Philip's War" seems a better first step.
Even Google.com searches like: "cotton mather" museum jawbone (about 130 hits) are not a bad idea as sources of leads to more official sites. Trying to limit that search to college/university sites: "cotton mather" museum jawbone site:edu may even be TOO restrictive if you want to trace obscure leads - it only gets 5 hits.