It depends on the size of the show. If it is a schooling show, she could probably just wear a white polo or dress shirt, white or tan breeches, and tall boots or paddock boots, depending on what she owns right now. Schooling shows are really informal. If it is a bigger show, I agree with rodeogirl- she could wear a hunt jacket, but if you don't have one, I would suggest buying a dressage jacket, unless she wants to get into hunters/jumping.
Dover Saddlery usually has discounted prices- Dover Saddlery - Quality English Horse Tack & Horse Supplies for the Equestrian.
I believe they also have outlet (?) stores around in some places, also.
You can also just google "tack shop in ____" or whatever to find a store near you. Check with her instructor (if she has one), also- they might know of a good tack shop with show coats/show clothing.