Ebay is the cheapest I have found for the 100 micron 4x9" nylon mono-filament socks. £7.99 with free p&p. There are also the 7" felt ones under motor stuff as they are generally used for veg oil filtering.
Coleparmer do them as well and they go to smaller sizes in the nylon than ebay and they do felt ones plus another I can't remember. They do the 4" and 7" rings.
Felt is less efficient than the nylon mono-filament. The type of material you pick is important to what percentage of pieces that are caught, ie a 100 micron nylon sock will catch 98% of the 100 micron pieces that flow through it. Can't remember where I read it but it was something around those lines. Nylon is also easier to clean and apparently lasts longer.
They work out very expensive here compared to the States but Mcmaster is not shipping overseas until further notice according to the reply I got. I was going to buy a huge box of them and sell them much cheaper than UK stores.