If you're driving in on Monday, check out Mamers for a great street market (cheese man likes to practice his English so will set you up with the best smelly stuff to put in your tent to keep the thieves out) and good place for a short stop. Just about 40 mins north of the great City of LM and a good alternative route if you come off the m'way at Alencon. Also, Bonnetable has a market Tuesdays. Biggest problem with both of these is that they interupt the serious business of scrutineering but if it keeps the peace....
The usual big street markets in Le Mans (Jacobins) tend to get interupted by something else that's going on there.
Marketdays in the Sarthe that I've found so far are : Mondays : La Ferte Bernard, Mamers. Tuesdays : Beaumont, Bonnetable, Bouloire, Ecommoy, Loue. Wednesdays : Ballon, Le Grand Luce, Sille le Guillaume. Thursdays: Conlie, Le Lude, Marolles le Braults, La Suze sur Sarthe, Tuffe. Fridays : Malicorne, Pontvallaie, Vibraye. Saturdays (well, if you're around the previous weekend) Allonnes, Brulon, Fresnay sur Sarthe and Montfort le Gesnois.