Wow. trying to come to Loire Valley from Paris on a Sunday and no rental car companies open in Amboise or Tours. Are there taxis available to get to our hotel? I am trying to get rooms at Chateaus de Nazelles
The Chateau will be able to tell you about taxis, if any. Alternatively, the Europcar office at the Tours train station is open on Sundays, but oddly only from 5:30 PM to 10 PM. You didn't say when you were traveling - I checked the month of May. Don't know if you'd consider this - but the drive from Paris to Amboise is easy. Takes about 3 hours. And you could easily take a side trip to Chartres on the way, to see the magnificent Chartres cathedral. I don't think you'd have much trouble renting a car in Paris on a Sunday.
I had the same problem. We were headed to Loire on a Sunday in July. We've opted to rent a car in Paris and then drive to the Loire. It's not much longer to drive than it is to take the train and without the worry of finding a taxi and not being able to get around when you arrive.