You can try emailing some of the pharmacies near where you will be staying. Search on Google Maps.

Or email the tourism office of Venice. Or ask your hotel if they know of places where you can get covid antigen tests 3 days before your departure.

There is a site that lists pharmacies which offer tests but then you have to search the addresses to see how far they are from center of Venice.

Also when you're walking around, you will see pharmacies with signs saying they offer covid tests or they don't offer covid tests. But if you want to plan ahead, a lot of pharmacies have websites and email addresses.

They typically require appointments too, can't just walk in.

They also offer take home covid tests but those will NOT be sufficient for your return flight. Instead, the pharmacies which offer test results for travel certification are going to give you a printout and a stamp of the phamacy showing their official license from the city and commune. Or a QR code in the case of pharmacies in France.