Vitamin D is added to most foods in the US, federal rules or something back a few decades. It's to help with kids when they are growing and the parents couldn't afford "healthy" food. Anyways, check the labels, almost everything has some vitamin d in it now. Most "food" items, not "junk" food
Get some sun too for vitamin d production.
my stance on supplements are that unless you have a disorder/reason you can't produce/absorb them, you'll get enough from eating normal food, not junk/fast food.
edit:
disclaimer, i do take fish oil pills, not religiously but when I feel like it :S. I don't "need" it anymore but it doesn't harm me either. But I needed them and vitamin b when I was on other medication in the past. Since I have them still, I take them when I'm in the mood, since I don't want them to expire. And I'm fairly sure taking them won't harm me either, not for taking a pill every other day or once a week :D. I'm just not getting the benefit from it either I think.