How to Dress Like Troian Bellisario
This one is actually very simple. Aeropostale has a line of Pretty Little Liars clothing including outfits matching things worn by Troian/Spencer on the show. There is also the very popular Emily line.
You may be unwilling to purchase a whole new wardrobe for the part though. If so, just find preppy, yet cute clothing. I would try a lot of blazers and tight fitted pants.