The first row of pictures shows the costumes from the first two Harry Potter movies. The second and third rows include pictures from HP & the Prisoner of Azkaban. Many times the pieces from these costumes can be purchased for a decent price online - I've included some links [here]. Also, the Madame Malkins Yahoo!Group is an excellent resource.

Linkage:
My HP University Student - my take on a HP costume as a university student
My HP POA Costume - my HP costume recreation from the later movies
Maggie's Harry Potter Costume - a great recreation from the first two movies
Starfire Phoenix's Harry Potter Costumes - both Gryffindor and Hufflepuff; also a great pleated skirt tutorial