According to the PostNL Locatiewijzer, there are 4 outlets at Schiphol where you can purchase stamps.
And indeed they all appear to be landside, rather than airside.
While I remember that there used to be at least one postbox airside at Schiphol, it appears that the only postbox actually within the airport proper is on Schiphol Boulevard, which I believe is landside:
(The number of postboxes in NL has reduced quite a lot in the last several years)