Marrow bones can be purchased at your local butcher. If you can, ask to have the bone split in half horizontally so that you can get more marrow and flavor out of it. To freeze the bitterballen, lay the breaded balls on a sheet pan or large plate in a single layer and freeze until hardened, then transfer the balls to a resealable freezer bag. When ready to cook, deep-fry the frozen bitterballen at a lower temperature (350 degrees F) for about 4 minutes so that the inside becomes molten as the outside slowly gets golden brown.