From my RV dealer. They can be ordered from Amazon.com also, but it's hard to tell which to buy for which furnace/water heater. I've actually not bought and installed them yet, during the walk through there was a leak somewhere and the outside grill was missing, so the dealer still has my A124S. Once that is repaired, I will buy and install the screens. If I have anymore info I will post it.
Jerry, Radio Amateur N5RV
FR Rockwood A124S
2013 Toyota Tundra Double Cab 4x4