Fisher Creamy Peanut Butter and Skippy Peanut Butter are available at Albert Hein.
Root Beer is available at C1000, Super Coop and Makro. Its pretty funny the Root Beer is in the beer area at C1000!
Oreo Cookies are available at Hoogvliet, Albert Hein, and Super Coop.
I've also seen american Marshmellows at Hoogvliet and the Lidol.
If the Lidol has a American weekspecials there are also different foods available...even jelly beans!
There is also the American Store in the Bogart Winkelcentrum, Rijswijk where they sell a few American food and candy supplies, however they are vey expensive!