If MERV 13-A filters are not available, the next filters recommended are MERV 13 filters from 3M . Specifically, 3M 1900 filters are excellent (the people on the thread are IAQ experts) .
These can gotten at:
- Home Depot
- Ace Hardware
What is not recommended?
FilterBuy’s filters sourced from Amazon. There have been knock-offs sold on Amazon, and so it is recommended to avoid those.
Last tip – if you are making the 4 or 5 filter box, you could make the sides out of 20″x30“x1 filters – and get a huge gain in filter area. You can find 20″x30” filters at Walmart. Tip courtesy of ocnj of Twitter.