Simply French Vintage on Etsy.
Simply French Vintage is owned by David and Jane, a couple from the UK who live in France. They have a number of these grain sacks available for sale in their Etsy shop, as well as other vintage French items.
This sack that I purchased is from the 1960s; there are a couple of tiny mends which add to its charm and authenticity. And it's large--27 inches wide, 48 inches long.
I've seen these grain sacks made into throw rugs, or into pillows; my current plan is to have it stretched and framed under glass and hang it in my kitchen after my renovation on a tall, narrow that will lead into the kitchen from the dining room. But if you have an idea, I'd love to hear it!