What kind of package / font should I use to get the following version of the capital N in math mode?

This is the standard notation for the normal sheaf associated to the divisor D.
What kind of package / font should I use to get the following version of the capital N in math mode?

This is the standard notation for the normal sheaf associated to the divisor D.
$\mathcal{N}_D$ should give you what you're looking for. You don't need to load any special packages.
$\mathcal{N}_{D}$. See How to look up a symbol – egreg Sep 24 '13 at 10:40