You could train troops and sell them in auction in the Tavern (Meyhane-> Ihale).

You will not get anything if no one buys them from you but if your price is low enough, you have a better chance to seduce the other players to buy your troops. You will get some gold for that (50% of the final price).

Otherwise, there is no secret: you buy the gold in TL!

Enjoy the game!