Check out your local fabric store for polyester voile material and nylon thread. It was about $5 for enough material for 3 large bags. You just have to have access to a sewing machine or have a lot of patience to hand stitch a bag.
Hopefully the large 5 gallon Lowes paint strainer bags are working out ok for you. I am going to come up and brew on your system to see how things are going and get that efficiency up.