The Hidden Tower Forums
We have an ever-growing EXTENSIVE list of links to both Sims 1 and Sims 2 Harry Potter themed downloads. 

Marina's Sims (Sims 1 and Sims 2)
Awesome windows and arches and things.

Simmington Square (Sims 1)
Best Sim houses on the web, including an entire Harry Potter neighborhood. Be sure to grab Hogsmeade!

Ophelia's Little Page About the Sims (Sims 1)
Tons of Medieval objects and skins.

Cheep Frills (Sims 1)
Awesome flying broom and sword fighting objects.

Harry Potter Sims (Sims 1)
Many teachers skins here such as Dumbledore, Hagrid and McGonagal.

3D Designs for Sims (Sims 1)
Thanks to Jerome for allowing the public to use his meshes for skinning. The best around!

Simfreaks (Sims 1 and Sims 2)
Paysite that has virtually everything you could want for your Sims. Had medieval theme which includes a medieval pub, and a host of other fabulous walls, floors, objects and skins that could be used in a Harry Potter neighborhood. An entire Expansion Pack for a few dollars a month.

8th Deadly Sim (Sims 1 and Sims 2)
Dragon and three headed dog skins! Adult pay site.