There are social media accounts that are dedicated to certain celebrities clothing. These fans find their clothes and prices.
Most Helpful Guy
If you mean the EXACT clothes they wore as in used probably stalking or extreme fan girl-ing etc or the same design of clothing im guessing their Instagram or other social media or word of mouth or sponsorship's etc0THIS IS NOT RELEVANT ANYMORE
Most Helpful Girl
they know because it's the top designer stuff. i can look at a celebrity and can guess the designer pretty often. like kendall jenner always wears shadow hill hoodies and you see people in things like valentino or dolce and you just kinda know. these people are super into fashion and they know what's out0THIS IS NOT RELEVANT ANYMORE