The blueprints would come from the builder.
As for floor plans, it's possible that a real estate company in your area might have them. In the office I work out of, we have maybe 8 file cabinets totally full of floor plans. They were usually collected from the builders at the time a subdivision was built. In some cases, they came from an original homeowner who held onto the developer's package of plans, and then gave them to his/her agent when he/she sold the house. And they can date back quite a way--there are floor plans in those cabinets going back 30 years.
Hope that helps.