George,
Well it depends on where you are located. In the USA, this type of stuff is readily available from industrial suppliers like McMaster-Carr. A link to their web page listing wire mesh is here:
http://www.mcmaster.com/#wire-mesh/And you can browse the material and mesh sizes available under "woven cloth", but for example the smallest openings for given materials are:
Bronze, 0.003" opening, 0.0021" wire, 12"x12" sheet, US$25.81
Brass, 0.006" opening, 0.0045" wire, 12"x12" sheet, US$7.48
Stainless Steel, 0.0015" opening, 0.001" wire, 12"x12", US$25.58 (finer available but it gets pricey)
The last you would have to epoxy on to your valves, but the mesh would be guaranteed fuel proof!
By the way, McMaster has no minimum order (no account required with credit card orders), and they do ship world wide. However the post would be cheaper if you could find a similar firm closer to where you live.
-Doug