There can be more involved in "exterior locksets" for French Patio Doors. We will need more information and possibly pictures to be able to guide you better.

I assume you mean single point locks? A french unit such as Marvin will probably come with a 3 point lock, which is specialty item tailored to that door.
Try a good Schlage dealer, or check them online. Baldwin also has an economy line that might also fit your needs.