A really good shop that I love going to is called Mid K Beauty. It's in the Gresham area, right next to a CD & Game Exchange, and across the street is a Fred Meyers. They're very helpful, and you can try on as many wigs as you like, though you have to buy the wig cap for a dollar. They refund your dollar if you buy a wig, though.
That is the first place I would recommend, and they sell TONS of wigs, as well as wig shampoos and things. :3