Round Hill, Purcellville and Leesburg all have markets. I think the Round Hill Market is on Sunday, but dont quote me on that. I'm sure if you were just to drive into town, stop at the local diners and ask you could find out.
If your free on Sunday, Shepherdstown WV has a nice little market. It's a pretty drive and there are a couple good restaurants there-Press Room, owned by a former employee of the bank, and the Yellow Brick Bank which is OK.
The Post publishes a list of markets in the food section. I havent seen it yet, but you could access that for a really comprehensive listing.
I lived in Martinsburg for several years and LOVED the Shepherdstown Farmer's market. Among other things, I would pick up a homemade scone, grab a coffee from The Lost Dog, head over the bridge to the C &O Canal, nibble my goodies, then walk it off.
The Press Room must be new. I always thought the Yellow Brick Bank was overrated. I may need a roadtrip to check it out:)
How far into Leesburg are you? The Leesburg's farmer market in on Saturday, across sreet from the police station. But I also go to the Sunday ones in Cascade, which is right next to the Cascade library, and behind the shopping center. Its 9-1, and has similiar vendors as the Leesburg, and Sterling market.
Here's the list of all Farmer's market in Loudoun County:
The problem with these markets is that the quality seems to be diminishing with the lack of farms in the area. Most of the produce comes from way out in West Virginia, so the idea of fresh local produce is a bit of a stretch. But it is still better than the grocery store which makes for a nice summer salad. There is some quality local produce grown outside Lucketts on 15 North near Lucketts. But they seem to be stocked mainly in late July and August. Thus if you're looking for tomatoes beware of what you get early.
Not much better than store tomatoes IMO.