Since they are already round they will stay round, they may get a little oblong if you don't rotate them enough.
In the wild real Marimo can grow in 3 forms; Ball, Flat or free floating pieces. They grow extremely slowly and rarely multiply. It supposedly takes something like 50 years for a ball to go from a single sprout to a 1 foot diameter ball. That's why real marimo are a protected species in every country they are naturally found.
If what you buy in the store is real it will be very expensive and probably bred from one that was imported ages ago.
As for white plants try wal-mart, that's the only place I've seen them. I realize you want them for your theme but be warned; they show algae, dirt and diatoms like nothing and if you treat with a chemical that changes the color of your water they will be permanently dyed. TL;DR They are very difficult to keep white.