Locate the builder. If your home was built within the last 5 to 10 years, you can probably get copies of the blueprints from the original builder or from the city planning office. If you aren't sure who built the house, take a look at neighboring homes. If they look similar, it's possible they were built by the same company. Ask your neighbors if they have this contact information.
Check for permits on file. If you have an older home, there's a chance it has already been remodeled in some way, and there are permits and possibly blueprints on file. Check with your local city planning office for permits that have been filed. If they do not have copies of the blueprints, you can probably get further contact information for an architect or contractor who worked on the home. If your house is a new construction, blueprints are likely on file with the building permit. You can get copies for a small fee.
Check with your local realtor. Find a local realtor who has been in business for a long time in your area. They are probably familiar with local builders and can help you find information on your home.
Check the deed transactions. If the house has been bought and sold, you can find information on previous owners. If you can locate the original owner, you can find the builder and get a copy of the plans directly from them.