You can use a Japan Rail pass on the Hikari, but not the Nozomi. Purchasing a JR pass in the US cost less than purchasing the same pass when in Japan.
*You cannot purchase the nationwide JR pass inside Japan. The other regional passes cost the same if bought overseas or in country.
You can work around this buy ordering from an overseas office and having it mailed to you in Japan (but is time consuming because you have to wait for mail)