There are a number of roadside stands on hwy 125 around St-Esprit (near where hwy 25 ends). (The usual Montreal–St-Donat route is up the Laurentian Autoroute; to reach these stands, take an alternate route through Laval along the 440 to the 25, then follow 25 until it ends. From there, the 125 goes straight to St-Donat.) [And if you like poutine, stop at Marco in Chertsey, roughly midway between St-Esprit and St-Donat.]
And while it's far from being a farmer's market, in past summers there's been a little stand in St-Donat that sells good local produce. It was (and I assume is or will soon be) on the west side of Principale, more or less across the street from the bakery IIRC.
There are Saturday markets nearby at Val-David and in Mont-Tremblant Village. The market at St-Jérôme, though a considerable distance from St-Donat, is open Tuesday, Friday and Saturday. Follow the link for more info.
Look out also for road-side farmers' stands along route 117 and other local highways.