I use Pinterest, it's great in suggesting you the similar vibes pictures. Don't forget you can take trendy looking clothes and just make them look more plain. For laid-back...try just looking through "Korean fashion", "Korean every day outfit", "Japanese casual outfit"... maybe something from 90's fashion?
But mainly I just created a "basic wardrobe" for my characters and dress them within it.
So they have like 2 pairs of bottoms (jeans), 2 pairs of shoes, 2 different jackets... and then I just add different t-shirts (usually I repeat those too) and a couple of sweaters, basically tops that you can change every time. And then you just juggle this basic wardrobe as you want, adding accessories if you need. I also chose primary colors that their wardrobe consists of, this really makes them stand out and makes your life easier when creating clothes.
Most people don't really have an overly huge wardrobe unless they are fashionista or shopping addicts 