kosher marshmallows where to buy?
Go to Brooklyn, Avenue J or Avenue M. Take the circle q (yellow) train. Any grocery store there will have kosher marshmellows. Williams Sonoma, no way.
In Manhattan you could also try Kosher Marketplace on Broadway and 90th or Mezonot on Columbus and 94th.
But brooklyn is your best bet.
A caution for the vegetarian among us: true "no gelatin" marshmallows appear to be no more. When I was younger, I remember agar agar being the coagulent, but all the (several) brands I looked at recently at the kosher supermarket on Coney Island Ave had fish gelatin. (Camping wasn't the same without them!)