Highly recommended floats:
Plane Fun Floats:
There are foam core floats that come fully sheeted with balsa. They have a hardwood spine built-in for mounting the floats to your model. They come in a wide variety of sizes and are very reasonably priced.
Highly recommended foam cores for floats:
Seaplane Supply Company:
There are very nicely cut foam cores for model floats. They are very reasonably priced and are available in a wide variety of sizes and styles. The owner, Mark, will also cut custom sizes, still at very reasonable prices.