First let me apologize that I MEANT to list Plane Fun Floats in my last message, but I had a "Senior Moment". I have now corrected.
Some of the great flying experiences I ever had, happened with Kadet Seniors. Three or four of us would play "Pass the transmitter" with a Senior, and whe it was your turn, the game was to circle the pattern and to perform the slickest landing possible. WOW! I improved my technique SO much doing that!
Ralph was right about the proper float size for a KSr being 42". That is what these planes have. I was the foam-cutter dude for that club when I lived there.
I mentioned alternate suppliers: SeaCommander Floats have a line of foam floats, and they have been around a long time. Slocan floats has something of a cult following in the PAC NW. Neither of those suppliers, to my knowledge, participates in this forum. Meanwhile, Ralph and Mark are right here with unbiased advice whenever somebody needs them.