Any of the available ".40 size" floats would work just fine. Lots of companies make them. And there is a lot of info out there about how to mount them on a typical trike-geared plane. You shouldn't have any trouble.

(thinking about it, I haven't looked at the Nextstar's gear mounting. If I recall, it's a kind of break-away rig, right? I'd assume it would still be strong enough to handle floats, but I don't know for sure, so don't blame me if you have to swim for your airplane ).