Don't worry about the weight. Get the float size by the length of your plane. Foam floats are pretty light.

Go to these guys- great foam floats. [link=http://www.seaplanesupply.com/]Seaplane Supply[/link]

or these guys. [link=http://www.planefunfloats.com/]Plane Fun Floats[/link]

I've used, and am using, both types.