You’ll get the shard randomly, fighting monsters is the most common but I think you can get it other ways, I would just keep mining, focus in getting the bus fixed (that’s the next step after you finish the mines, which it sounds like you are close to) and let yourself get it randomly.

Seeds are definitely your best bet for money early on. It’s kind of your choice for barn vs coop upgrades, you need both eventually and they each give you about the same extra money. Make sure to make mayo machines and cheese machines as soon as you can, that will greatly increase the value of your animal products.