You can use https://www.skyscanner.it and https://www.momondo.it/ to find Italian OTA's selling the flight you want.

In addition to https://www.expedia.it/ there is https://www.edreams.it/.

https://www.prenota-voli.com/ and https://www.volagratis.com/voli/ are also well known.

I cannot attest to these but here are some others:
https://www.piratinviaggio.it/
https://flyuvet.it/
https://www.opodo.it/voli/
https://flighttix.it/
https://www.budgetair.it/
https://www.lol.travel/it

However, personally if I were utilizing this rule I would absolutely book directly on the carrier's Italian website or via brick and mortar TA in Italy. I would avoid adding another layer of complexity by going through on OTA. I have had to fight OTA's to comply with fare rules in the past and it was not a fun experience.