With the Ultra Stick you have a light wing loading, huge control surfaces, and flaps. Even doggy old GeeBee floats will work.
However, if it were me, i would use a nice light set of foam floats about 36" long ( you could get by with 32s but the prop might be in front of the floats) There are several foam float sources listed on my website (below) and one I mean to add to the website is www.geocities.com/planefunfloats.
I flew a Jerry's Little Boy (Similar to the UltraStick) for three years on BJ foam floats, and it was completely successful. BJs are gone now, but the PlaneFun floats are very similar.