Buy it off other players.
It varies according to the stock.
Please do bear in mind that stock does very little at the moment. It allows you to view some extra data about the faction that it pertains to, and 51% of it equals ownership. However, there is no automated system that will provide dividends from stock, and I'm not aware of any factions that do it manually.
At the moment it's mostly just one more thing that can be traded around.
____________"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
Only fools and children dream of heroes.