Some of the screens in our windows still have some of the weatherstripping that was apparently originally installed. In cross-section, the stuff looks like a "T" atop an "O"; the top bar of the "T" fits into a groove around the perimeter of the screen, the "O" squashes to bugproof the edges.
I can't find this stuff ANYWHERE. However, the mosquitoes and other undesirables are able to find the spots where the weatherstrip is missing.