I have an unusual source to add: as these sieves are used in
(silk-)screenprinting, I get those nylon-sieves from a local supplier
here in Austria (would be of no use to add the name and address, I
think).
They come in different mesh-width, and different material (nylon,
polyester, metal, and so on).
I use these (nylon) for flowcytometry, but they are also used in
tissue-culture (so they can be sterilized), and come by square meter
(last time the price was about US$ 100 per sqm, and this is enough
for about 10000 runs).
Maybe this is a cheap source, I think it is worth you check this out.
Hope I could help you
w.w.
Mag. Wolfgang Wlach Institut fuer Pharmakognosie
der Universitaet Wien
wolfgang.wlach@univie.ac.at Pharmaziezentrum
Tel. +43/1/313 36/8270 Althanstr. 14
Fax. +43/1/313 36/772 1090 Wien