Hey...I've built a few vivs and yes, the 40 Mesh is what your after. Cheapest place for only a couple square feet can be found on ebay. I'd recommend a seller by the name of "compasswirecloth".Well, today I've searched for "stainless" and found plenty of results.
It seems that a mesh count of 40 (wires per inch) is enough to keep FF in.
Thanks.