I'm assuming you don't mean the Olsen twins clothing line but instead, the clothes that Mary Kate actually wears. She is very high fashion - known for being "boho chic"
I would try small boutiques because they tend to have more unique pieces of clothing - but you could shop just about anywhere. Even vintage stores!
Pair some skinny jeans with an oversized top and some knee high points. Pull it all together with an oversized brightly colored bag. All these things can be found at the mall, especially stores like Urban Outfitters and H&M
