Never underestimate the effectiveness of weathering your own burlap.
A few years ago, I bought a few different shades of burlap from JoAnn's (early August) for a witch I was making.
I decided to let it sit in the sun for a couple of weeks (and the dew and whatnot - no rain in SoCal in the summer). I made a point of bunching it up and letting it sit for a week, and then unbunching, re-bunching, and letting it sit for another week - I think I did this for either 3 or 4 weeks on a shelf in my back yard.
Ended up with some wonderfully discolored, aged burlap. Only a couple spiders to shake out after all was said and done (once again, was on a shelf, not on the ground).