Is there an outdoor farmer's market in Pavia? Where and when?
Every Italian city has a green market, which is usually located in the city center. Some farmers sell their products, mostly fruits, in front of their farms on the local roads.
Plenty of markets in Pavia:-
Piazza Petrarca from 08.00 - 19.30 (Wednesday and Saturday
Piazza Petrarca from 08.00 - 12.30 (Monday, Tuesday, Thursday and Friday)
Via Olevano from 08.00 - 12.30 (Tuesday)
Via Manara from 08.00 - 12.30 (Wednesday)
Piazalle Torino from 08.00 - 12.30 (Saturday)
Via Aselli from 08.00 - 12.30 (Thursday)
Piazza Marconi from 08.00 - 12.30 (Friday)
The markets are not 'farmers' markets' in the sense I think you mean but just normal fruit, veg etc markets where stall holders *might* be the actual growers but more often are not (which does not mean the food on sale isn't produced locally, of course).
All Italian towns and cities (and larger villages) have regular markets of this type.