In the game or in real life?
In the game you don't need to buy them new clothes! Just go to the dresser and select plan outfit to redress them! Or if you have Ambitions you can see a stylist for a makeover (the stylist picks the clothes and everything though!)
In real life you can buy stuff packs, expansion packs, and individual outfits and hairstyles in the stuff store at thesims3.com
