I called all the major dealers, they're very expensive.
where can I get generic liners, bargain priced?
Do you mean the fabric over the tube? You make one with a sewing machine and felt.
I would never recommend felt. I know it has and currently is being used, but its too rough IMO.
Buy a yard of something like this.
http://www.joann.com/signature-seri...ml?green=41C4BB9E-FC55-58ED-0CE9-BA6AA3FF117C
The irrigation pipe from above is pretty standard. So for that size pipe, you will need to cut the fabric into 4" strips for the shaft pipes, and 6" strips for the butts 42" or so long . Fold them face to face, and sew them closed.