My travel mate has struck gold, uncovering a really long list of markets to be held in Lombardy this year. I can't vouch for its accuracy or for the quality of the markets themselves, but there is at least some indication of the type of goods being sold:
https://files.spazioweb.it/62/8b/628b5a9d-4a02-4c04-8bb3-0cd23ba0ee66.pdf
I believe the list is compiled by a market-management company, so markets managed by others are not included. For sure there are farmers markets not listed here. However, I think this represents a substantial share of the antique/vintage/artisan/collectibles markets in the area.