Many thanks to those who replied below; I called Truevalue Hardware and they have 1/8" mesh, which I hope to get tomorrow.
While I wait, and since it was in the 70's today, I opened the hive and saw the top frames covered with bees that seemEd to enjoy sunning themselves. So I went and got a magnifying glass (5x) and studied them carefully for quite awhile. In all the inspection, I was unable to see a single mite. What are the chances 1) They're there but I didn't see them 2) There aren't any or too few to be concerned about 3) something else.
I am still going to begin monitoring, as I'm not willing to be surprised, but I was interested that I couldn't see a single one.
By the way; on the origin of the hive; a friend upstate gave me a hive from his splits last spring. I got it home and set up, and the queen only laid one frame of eggs and then the bees commenced a replacement which I let go. The present queen is laying in beautiful full frames, and the bees are very tame so I have no idea what the background really is but it seems OK now.