That's a lot of tile to find stashed in someone's garage. Usually if people have extra for their own homes, they want to keep it in case of damage to existing installations. It would also be rare to have that much overage. I was able to find some matching tile once for renovations on a family member's house from a Seconds and Surplus store in my area. You might try places like that if you think the tile would have been a common one used by builders. You also might consider exchanging more than just the 50 square feet to a natural shift in room delineation and getting something that blends.