One way is to buy them ( I presume with a credit card?). Sometimes there are competitions which award gold bars. I have earned over 50 gold bars in the last couple of months by entering various competitions. Keep your eyes open, as they are available for most King games.
The only way I have ever found on this game to get gold bars was via the Treasure Mill event. This hasn’t been available for over a year now unfortunately. Hopefully it comes back soon