Welcome to the forum Glyn123.
Polyester is easy to find much cheaper than that. Go to a yard goods store, one that sells cloth for sewing, and look at the huge bags of polyester for stuffing pillows. Avoid the more expensive polyester that is labeled as fire retardant, you don't need the fire retardant in your fish tank's water. As long as you do not need a specific shape, the pillow stuffing makes a great polishing filter right close to the filter outlet.