If you split them, they grow into balls slowly again. You can buy some off ebay for $16 shipped for 2 balls. But the guy lives in Malaysia I think. So thats why I wouldn't buy from him.
I also want some Marimo balls. Can't find them anywhere either.
I grow lots of clado-algae. Maybe I should start making balls... I'm thinking just wind the stuff around the ends of my fingers like you do w/ yarn and see if it seals itself...