I don't need to winterize (fortunate me, I only have to drown-proof my plants), but all gardeners need to know how to find good materials for little, ideally no, money.
Allow me to introduce you to Freecycle.org, the world's largest swap meet. Freecycle.org was founded almost 10 years ago (I think in 2004, but that's a SWAG) by one man in Arizona; it has now grown to over 9,000,000 members worldwide and over 5,000 local groups. The mission is to "keep useful stuff out of the landfill."
I responded to an "Offer" a couple of years ago, when two burlap bags were offered on a list I'm a member of. These went on top of my compost inside the Biostack. They eventually disintegrated (decomposed).
Why not try it? Go to https://freecycle.org and do a zip-code search for the U.S., and sign up for the list(s) which seem most likely to serve your neighborhood/town. Put up a "Wanted" announcement and see whether anyone has burlap bags hanging around that they'd like you to come and pick up.
It's pretty amazing. It doesn't always work, but it works often enough that I recommend it for most things like this.
Good luck!
Cynthia H.
Sunset Zone 17, USDA Zone 9
. I have seen sites that say to wrap them in burlap but have found it would cost more then I would like to spend. 