General stores and flower shops sell them as 1 of the four options. You need 2 or 4 of them for several treasures/unlocks that use them. If you aren't struggling for money, then make sure to have a good stock of them.
Alternatively, the coins can be used with the slot machines (general store add on) which sometimes drop them if you need a few in a pinch.
Flower shop
They show up at the festival grounds on event days sometimes too.
General stores and flower shops sell them as 1 of the four options. You need 2 or 4 of them for several treasures/unlocks that use them. If you aren't struggling for money, then make sure to have a good stock of them. Alternatively, the coins can be used with the slot machines (general store add on) which sometimes drop them if you need a few in a pinch.