I bought a small, portable fish finder and it came with a round, plastic disk that keeps the transducer from sinking.
The best place to check for an aftermarket float would be (don't laugh), your kid's toybox. My kids have plenty of plastic floaty round and flat hoobajoobes that could be modified. Just make a small cut to get your line though, silicone it shut and walla.
Now, to just keep the kids none the wizer as to where all their toys went.