For years, in our current house, we had an odd-ball size air return vent for the air conditioner at one end of the house (we have 2 units on opposite ends of the hosue).
I absolutely could not locate filters, so I made my own. I purchased filters that were larger and simply cut them and taped them back together. It worked well, and I never had issues ... until the furnace died completely.
Then we had a new unit put in. (our coils are in the attic, as is the return). So it was so old of a system, it meant replacing a lot of stuff. The return vent was one thing that got replaced. It took some frame modification with our attic rafters, but the end result was a 14 x 14 inch return. Oh boy! Guess what? 14x14 is hard to find also.
Thus.... enter Amazon! Problem solved.
Check Amazon. Amazon has everything!
Here's a note on "MERV":
MERV stands for Minimum Efficiency Reporting Value, which describes the efficiency of an air filter in reducing the level of 0.3 to 10 micron-sized particles in the air that passes through the filter.0 A higher MERV rating means that the filter can trap smaller air particles.12 The rating is derived from a test method developed by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) and comes with a range of 1-16. The lower the number on the MERV scale, the lower the filtering efficiency will be. The higher-rated filters are capable of trapping smaller particles than the lower numbered filters. The MERV values can be viewed through four tiers.
So, why not got with one that has a higher "MERV"
Bingo! Click here.