Take the pully to any mower shop that does repairs, engineering supply shop or bearing shop.
All of them will be able to dig us a replacement of some sort.
Usually you will need to swap over the stand off bush from the bearing.
If the pulley is in good condition you can do as Dave mentioned but it is a lot of work, I have done it in the past myself
Occasionally if there is a common ( less expensive ) one available then I fit it and adjust either the belt length or tension arm to compensate for it .
A lot of my customers mowers now run much larger idlers because the bigger the pulley the slower it spins and the slower it spind the longer it lasts.
Also the bigger the pulley diameter the longer the belt lasts
