The Museum of the American Indian (a branch of the Smithsonian) located in the old
Customs House in lower Manhattan - right across from Battery Park - has a resource
room/library which is open whenever the museum is.  They have a reasonable amount
of data on this subject, though not, unfortunately, the Strong book.  Sorry not to
have the URL to hand, but they maintain a website which gives such elementary info
as how to get there and the times they're open.  Any of the search engines will
doubtless turn it up.
Beverly Martin