How can you find blueprints to houses or buildings?

Depending on the age of the house and the requirements and the time that the house was built you could check with your locality's recorder's office. I know where I live you're required to have a copy of the blue prints on file there for every house that is built.
Do you mean actual blueprints or something like a catalog of a bunch of houses w/floor plans? You can get catalogs from places like Home Depot or Menard's
A lot of times you can get floorplans to Mobile homes they seem to have larger bedrooms.