you get free 100 stamina at 5am and 1pm server time, up to 350 stamina
stamina pots can be bought in the black market (when they appear), for free in login rewards, in event rewards, given free in missions, or if you're really desperate you can buy stamina with ingots
One of the few and best ways to get stamina pots is Lucky Board, each one roughly costs 400 coupons to get, but most of the time they are just a bonus to what you're actually trying to get