It might not quite be Miskatonic University, but the University of Massachusetts at Dartmouth does have a collection of issues of the 19th century magazine American Architect and Building News. From that they have made a special collection of examples of drawings and floor plans of house and building designs.
These could be very useful images, both because the simple ink drawings are very atmospheric and also because the floor plans do most of the work of providing a map to use. Print out large versions of them, touching up the diagrams, and start adding your story.
A pair of large houses, via UMass Dartmouth
As old and romantic as they look, these are not the kinds of buildings that Lovecraft is forever talking about (in The Dunwich Horror, for example). Those houses are generally the starker, simpler Colonial style, the “gambrel roofs” that feature so prominently being the arched roofs you would see on a Dutch barn. These designs are more recent than them, and are harking back to an even earlier time.
But if the colonial houses suggest the decay of isolated, impoverished people, the Gothic style suggests the madness of excess and indulgence.
Two more houses from the UMass
Apart from the numerous large homes, the collection has several other kinds of buildings, ranging from small cottages, various public buildings (fire stations, train stations, churches), and even a couple of asylums.
A cottage and an asylum, each from the UMass collection