Where can I buy this doorbell button?
I would doubt it that you will find the exact replacement.

All doorbells that I am aware of, are wired pretty much the same, 2 wires, doesn't matter which screw it goes to. You will be able to find a doorbell that may fit the same drilled holes, that chance is better than finding the exact same doorbell.

Any big box store, or local hardware store will have a selection in stock.