I probably shouldn't publicize this, because then everyone will want them, but winter is a good time to get burlap sacks from coffee roasters. People normally want them in summer to use in the garden.
You need to go to the location where the shops actually to their roasting, not just where they sell the coffee. Go to your favorite local shop that has their own brand, and ask where their roaster is located. Then go there - or call - and see if they have any sacks laying around. If they don't have any now, ask if they will save them for you.
Last time I went by - at the end of summer - I took home about 15-20 bags, so they are out there if you know where to look.