Seasonal store or bank when offered. 

Possibly competitions?

Not available in store or in bank - where can we get these now???

Things change, and sometimes things change back.

nuggets are currently in the seasonal store, and so must have been when you posted.

Admittedly, not much use if you have no coin.

Coffer of the Gods have gold nuggets. You get Coffer of the Gods by emptying lv 8 and 9 tiles.

There's  also  some for Paladin 53, but again not much good if you cant get there, or have, and spent them.

Depending on your town level they can appear as stage rewards in individual events. My town is lvl 36 and gold appears in some events still. Just click on All Rewards button in the event to see if gold nuggets are available to be won.