Hi Gordon-
There may be several sources for these, including Beaver Coach Sales parts dept. They are made by Atwood, and you remind me I need to replace some that have come off of our coach. By blocking the window weep holes, they purportedly can block out 70% of wind noise. Until you get new ones, you can always move those from the rearmost windows up to replace any missing from the driver's and passenger's side where road wind noise is most noticeable.
Here is one source, at something over $4 for a pack of three, probably plus shipping. Keep in mind that these come in left and right coach sides.: http://pdxrv.com/catalog/i2729.html
and http://www.pdxrv.com/catalog/i2730.html
Make sure before ordering that these match the remaining ones on your coach because weep hole covers come in more than one type and fit different window makes, e.g. Hehr versus Atwood. Pinch the ends in slightly to remove or install them. You might clean the weep hole areas and then add just a dab of Lexel at one end when reinstalling each cover (don't plug the vent!). This should help retain them so they don't fall out easily again, yet allow removal if the holes ever need a clean-out.
Joel