What is the purpose of kosher salt in peanut butter? Can it be replaced with something else?
Alton Brown just generally prefers kosher salt, for reasons that don't really apply to peanut butter, which will be ground down anyway.

What matters is the total weight of salt.  Remember, kosher salt tends to weight approximately 1/2 as much (depending on brand) as table salt, per unit of volume.  

So you can replace the kosher salt with sea salt, or any locally available salt on a weight per weight basis.  The absolute amount of salt to use is also completely a matter of taste and preference.

The purpose of the salt in peanut butter is only to enhance the flavor.  It does not participate in any chemistry, and is not sufficiently concentrated to have any preservative effect.