Yes, tehy are aftermarket parts. Officially they are only meant for the racecars, supercars (288 GTO, F40, F50, Enzo) and special editions (Challenge Stradale, 550 Barchetta Pininfarina), but they were an option since I believe the 348 for the normal cars as well.
On the 308/328 they are stickers, but they can be in many forms, I also saw them in think plaxtic and even in metal so they are more realistic. I just remember they do not have to be stickers, I read also about magnetic shields so you can take them of the car without damaging the paint.