no you don't have to buy clothing from the store also depends on if you have any expansion packs or stuff packs if you have any of the ones that came with clothing use the clothes from those packs you can change the clothes the sims has by using the dresser click on the dresser-plan outfit then select the outfit you want and or recolour/use a different pattern from the in game create a style and the outfit can have a whole new look about it
there are 3rd party sites that has what is known as CC items but some of those may not work right or affect sims and or sims end up looking weird in some case's when sims had babies the babies looked like sticks and can cause issues with your game
and for the styling staion mentioned from the other poster you need ambitions expansion pack
