Northern Tool offers the ALECO AirStream Perforated PVC Strips 16-Pack for $184.99. Coupon code "254876" cuts it to $164.99. With $28.99 for shipping, that's $35 off list and the lowest price we could find. They feature 3/16" holes that allow fresh air to circulate while keeping flying insects out of the room.

Note: The mounting hardware is sold separately.