Wo bekommt man ein Grillset für den einmaligen Gebrauch?

Most grocery stores have them

Just use a primus and a gas container and bring a pan. Less of a hazzle.

There is a general campfire ban in Norway from April 15 to September 15, which also includes disposable grills in the wilderness. Outside this period, or if there is snow/a lot of rain (which removes the fire hazard), you can use disposable grills, but you are always responsible for fire safety. Use established sites, avoid flammable surfaces, and always extinguish the grill completely before leaving.