I tried to look this game up on Steam, but it says it's coming summer 2019. Where else can I find it?

It was part of the Humble Monthly bundle for April. I would guess the game will release on Steam and Itch.io sometime in May.

The game was the Humble Original of the Humble Monthly but it's no longer obtainable. Though, it's also a part of the Humble Trove now so it's probably the best way for you to get the game if you don't want to wait some months. Link Humble Trove

Now you can buy it on Steam :)