A lot of places carry 1/2"...he's looking for 3/8. Depending on the setup, mice & ASFs in the right size range can get through 1/2 mesh.
So far, the only place I've seen recently that has 1/3" (what I'm looking for, and possibly close enough for your purposes) is fairly expensive...
http://www.twpinc.com/twpinc/product...FUa8KgodPC2tFA
(I know I've seen it someplace else before, but a quick search this morning didn't locate it.)