Authentic farmhouse floor plans from 1834 to 1935 are available at HouseMouse.net. These floor plans are reprinted directly from original pattern books, and as of 2015 several are free to download.
The floor plans at HouseMouse.net include French farmhouses from 1834, along with barn and outbuilding plans. Additional plans are available for purchase. These are strictly floor plans, not blueprints.
Other farmhouse plans are available in blueprint form. These plans are based on old farmhouses and are not reprints of the originals. HistoricalDesigns.com has a large selection of farmhouse plans, mostly from the Victorian era. Southern Living sells farmhouse blueprints at HousePlans.SouthernLiving.com. The site offers a convenient filtering feature which locates blueprints by the number of bedrooms, bathrooms and square footage. ArchitecturalDesigns.com has several farmhouse plans, including traditional, Southern country and country-style farm homes.