Poly liner is a heavy duty plastic which you can buy and insert between the spring leaves. If you can insert them the full length so much the better.
On my 49 I found that the front and rear edges of the upper spring leaves were wearing notches into the lower leaves so I only put them plastic where they rub. I never took the springs apart and if I had I would have run the full length.
As to obtaining the poly, it could be had at a few places but what I did was find a child's roll up toboggan called a "Magic Carpet" at a garage sale and cut 1 3/4" strips from it. The "Carpet" cost me 50 cents.
