Not everyone dresses the same way IRL. When designing clothes for characters no matter the genre, try giving personal style a bit of thought. If a girl is more conservative and sweet, you're unlikely to find her wearing a leopard fur coat and fish net stockings out in public. If she is a bit of a tomboy guitarist, she'll be in jeans and a t-shirt with her favourite band name printed across the front. Unless your characters don't change regularly (like some journey, fantasy stories), they're unlikely to be wearing the same pair of jeans and t-shirt everyday.
But if you're OK with designing clothes for different character personalities, and the only problem you're having is with coming up with variety - there are lots of great fashion references out in the web. Pinterest and fashion blogs are some good places to look, just jam in a keyword eg "punk", "vintage fashion", "nautical fashion", "japanese street style" etc. For genres like fantasy, history and scifi however, I recommend looking into books as well, since there's a lot you can't get online.
You may need to do a few sketches of your characters in different clothes to see which one you want to use. Unless I have a very specific outfit in mind, I'm usually not able to come up with something on the spot. I definitely recommending testing out some outfits until you're happy. You might even come up with more than one you'll like to use in the future. Remember you can always change your characters' clothes if you decide it doesn't look nice after all xD