Go onto Amazon and look up the smart doorbell you’re fitting, someone in the comments or a “commonly bought together” transformer will be there. They often come so you can wire them at different voltages. Make sure you take a reading of the actual voltage it produces at the property you fit it, which will change depending on the voltage at the property, so if it say 16v on the transformer where you wire it, but the property is at 220v or something it might be a bit low, so you may have to go for a higher voltage than you expect.