Can Exchange Tickets be earned without spending real money?

Magikarp Jump has an in-app store for purchasing Diamonds and Exchange Tickets with real money.

As far as I know, Exchange Tickets are the only way to obtain certain items, such as the Diamond Miner.

While it's possible to obtain diamonds naturally by playing the game, I haven't found a way to earn Exchange Tickets without spending money.

Is there any in-game way to obtain Exchange Tickets without spending real money?

From the Prima Games guide on how to get the Diamond Miner, Exchange Tickets are, indeed, a premium currency and can only be obtained with real-world purchases.

Unfortunately, the only way to acquire the Diamond Miner is to unlock five levels of rewards with Exchange Tickets, which are a premium currency that can only be obtained by purchasing Diamonds from the Exchange Office. When you purchase Diamonds from the Exchange Office, you also acquire that same amount of Exchange Tickets, which can then be used to unlock different reward levels, each of which cost 500 Exchange Tickets. This means that in order to unlock the Diamond Miner, you will have to purchase a total of 2,500 Exchange Tickets—which will cost you roughly $45-50 USD.