I have received random brown stones from submitting business order and mastering businesses as well, so that's pretty easy to do.
Btw, I keep mastering businesses regardless if I have land space cos I know I need the mastery stars when we get more land like now. I just master and store them until I get more space. Hope this helps.
Edit to add:
You only need to do these 10 orders once and collect the rest of the stones from business orders.