Fortune Shop
When walking around on the Normal Map, you will sometimes see Iyami standing around. One Iyami tile is always working as a goal, which one currently in effect varies and depends on which prefecture Hatabou gave you when you spun the wheel. All the other ones will function as Fortune Shops. When you enter a Fortune Shop, Iyami will appear and speak to you. 
He will then offer you three fortune slips to choose from, showing you the backs of them.
You’ll pick whichever one you think is the Excellent Fortune (red fortune slip with the Mixer God on it) and if you’re lucky, you could score 5 Golden Coins. Keep in mind that different Fortune Stands have different prizes so even if you get the Excellent Fortune, you are not guaranteed to get Golden Coins.
A suggestion is to try and find a Fortune Shop that offers Golden Coins as a prize for getting the Excellent Fortune, and then walking back and forth to that same tile. By doing so you’d be able to try over and over for as long as you have dices, and could end up with a great fortune if you’re lucky.
You can tell if the Iyami is offering Golden Coins as a prize at the top left of the screen. 
If the 大吉 (excellent fortune) says “…G松コイン５枚GET" then you’re in luck, this Fortune Stand offers Golden Coins as a prize for the Excellent Fortune Card.
Keep in mind that it can be pretty difficult to get the right card as you have three options that are completely randomized, but keep trying, you’ll eventually get it!
I recommend this one particular Iyami tile (愛知 Aichi) for grinding Golden Coins.
This one is recommended because he is positioned in a very narrow path which makes it easier to land on, and there are no Quiz Hat tiles in the way. The con to this location is the Questionmark tile, which may cause Dayon to send you off to somewhere else, if you’re unlucky enough.
You can find him where Choromatsu is shown standing here, in the 中部 (Chūbu) region. 
All the Golden Coins awarded to you by this method will appear in your Gift Box.
Have fun and good luck taking all of Iyami’s sweet Golden Coins!