I don't believe the doorbell need a 24v AC, I am not even sure if 24v AC serve any purpose or exists. What you want is probably DC 16-24V, which you can easily obtain by using a simple power supply similar to laptop power supply unit, not a transformer to transform the AC current.

Edit:
Something like this:
https://www.amazon.com/SUPERNIGHT-100-240V-...r/dp/B00LUIHZZE

This post has been edited by Xaphier: Jul 21 2017, 12:41 AM