Ryan,
I don't think anyone manufactures mirrors specifically for amphibians.
What I've always seen used is a convex mirror from NAPA auto parts (~5 to 6 inch diameter) attached to a folded piece of aluminum stock, cut to fit the mirror, and bent at a 90 degree angle to attach to the wing. What we always did was attached to an inspection cover, so we could always remove easily if need be.
You can get away with only one mirror, IF you still want to rely on the gear indicators on the left float. The rub with the mechanical gear indicators is that you can't see the right hand side easily.
If you only install one, put it on the right side, and experiment with placement to provide the best view of front and rear gear. If you put it outboard far enough, and your vision is really good, you might be able to see the left gear with it as well.
My preference would be to mount two mirrors, one on each wing.
You may want to take a spray can of bright dayglow orange or green paint, and paint the side of your tire/wheel that faces the mirror. Ugly, but they are REALLY easy to see then.
I like mirrors, and as Sheephunter says, if you're not at least a little paranoid about amphibious gear, you should be. Get all the help you can to verify gear position.
Where the mirrors really help is if you were to get a gear in transit condition (which I've done a couple times). Question is: Do I land on land or water. Generally, gear in transit suggests land only. Mirrors may be able to confirm position of gear. If its even sticking out a little, land on a runway.
MTV