Steve,
If I could not get coir, I'd use a piece of heavy-duty short-nap rug/carpet as a replacement.
I had two original padding/upholstery seats in my Suburban. Besides the coir, another interesting "feature" was the burlap had metal strips/stays that ran lengthwise (about an inch apart as I dimly recall).
The coir and the "reinforced" burlap seemed to spread out the weight of the occupant. Without those items, you'll feel the springs.
