I have actually seen Hannah Montana Wigs orderable online but have very high doubts that they look even half as decent as they do on the box pictures they show. As far as other costumes don't be shy about going into retail stores or even halloween specific temporary stores and scout out costumes.

I dress on a very limited budget and have had my eye on a fairly decent look french maid costume for 20 bucks in the Halloween section of Kmart. Halloween is the most relaxing time to pick up a few naughty looking outfits.