Many times it's from the fashion designers. As mentioned before(so that the designer can get exposure) they usually give the celebs the clothes for free... I think that is so perverse, people that are poor and dont even have a penny to their name, hardly get any breaks, have to beg and get second hand stuff and here, these celebrities that can afford it all, gets it for free. just my pov.
they probably get clothes from ppl that want them to advertise for them and depending on where you shop, places in taiwan and china have some really really nice stuff for a more then reasonable price. too bad a lot of that stuff is hard to find in america -__-
they have company sponcer their clothing. they just have to go to the stores and pick the clothes out.
lol hey you know~ in Mong Kok there's this little mall on the side where all the stores are of japanese home made clothes. They are super cool and there are only one of each.. that's why you cant get those clothes so easily. xP. alot of stars go shopping there because of it's uniqness.
when they go to events, the clothing and jewelry are sponsored/by designers, in other words free. when they are going about their daily business they are dressed in clothing they paid for. u can buy these clothing urself as well, they don't cost that much. abt $100-$500 for t-shirts and day dresses. u can find them at these many shopping sites as well, http://www.net-a-porter.com/am/Home.ice