BINGO! I had the exact same problem today and solved it cheaply.
I pulled my Coleman AC cover off to remove the filters to clean them. YUCK, they were pretty nasty! Well, one ripped before I even got it out. The other one was pretty fragile too. I had seen them at a camping store at an out-of-sight price. Saw them mail order at a ridiculous price too.
Called my friend to ask him, since he does AC work and he said I should be able to find a small roll of the stuff at Home Depot or Lowes and cut my own. My unit uses two 16 x4 inch strips.
Well, another friend took me to dinner tonight, and I asked him if there was a Home Depot or Lowe's nearby I could run in. We ended up at Lowe's. I looked all over the filter selection and found a 20x30 1/2 inch thick one to cut yourself, but it looked like it would only catch the big debris. But I tossed it in the cart, to mull over.
I kept hunting for the thin foam ones. What I found was a package of twelve 4x12 inch Vent filters that help control dirt, dust, smoke and pollen for $3.97 so I tossed two packages into my cart and kept searching. My AC friend had warned me that the foam one you cut yourself, might be stocked some place weird.
Yep, I kept hunting, and I found them in the plumbing section. But the big thin foam ones were 24"x 15" and were $1.97
Gee wiz, I needed 4x16, so that is going to be lots of cutting and lots of leftover little pieces.
I tossed that in the cart to think over, while I hunted for something else obscure, which I couldn't find anyhow. Then an employee came along and tried to help me, but honestly, he was clueless. I began discussing the filters and the sizes and so on, and his eyes just glazed over, I could tell he was sorry he asked...
He was pushing the 24x15 and told me the missing inch wouldn't matter, but I was thinking it would matter, why filter the air, if you are going to leave a one inch gap? Meanwhile my cart was filling up with "what ifs". I often do this, fill up a cart of "maybe's" and then ultimately, sort through it all and only buy a few items or no items.
Finally, I settled on the two dozen 4"x12" and put everything else back.
Mind you I needed 4x16 inches. But I liked the look and feel of the 4x12"s over all.
Made it back to my motorhome, where puppy was thrilled to see me. I was so relieved the motorhome was in one piece too. He had not done a thing, except drag his toys out everywhere. It was like a mine field, of stepping on squeaky toys, and kicking tennis balls, baseballs, footballs,and golf balls. Sheesh, if all THAT keeps him from messing with my shoes and stuff, then I am happy too.
Um, back to the filters, I cut one up into three 4 inch sections, then used Scotch tape to connect the 4 plus 12, so now I have 16 inch filters, 4 inches wide, the exact size.
Which, last time I cleaned my old ones, I noticed the overhead Coleman unit was missing several of the wire clamps to hold the filter in place. Well, the old salt in me, having lived 22 years on small boats in far flung ports, I am accustomed to saving useful things and making creative repairs.
One of the things I save is the plastic coated wire ties from breads and from coiled wires, like when you buy something with a charger, it usually has a long wire tie around it. I save those things.
So I used the longer wire ties, to loop around the filter and the grids. The filters are held firmly in place. So I am good to go, and I have spares too. Let's see, overall I ended up with 18 filters total after, converting the two dozen 12 inchers to 16 inchers. Total cost under $8.
My unit uses two at a time, one on each side, so that is 9 changes,less than a $1 per change, far cheaper than the ridiculously overpriced ones I saw in an "un-named" camper supply store. They appear to be washable. They claim on the package, to change them every 90 days, since they are white, I figure you can see when to clean or change them. Even if I use the AC year round (and I don't) I guess I might have a supply for over 2-3-4 years now.
Hope this helps!
By the way I am currently at Paris Mountain State Park in South Carolina, $20 per night includes water and electric. Sites come with a fire pit, grill and picninc table. The campground has restrooms and showers, though I don't need them. There is a dump station here. It's beautiful, so lovely, I've keep on paying and staying, I've been here, um, a week or more. I've had to move 4 times, due to other camper reservations and my lack of planning, but I don't mind. The park also has a swimming lake, fishing, hiking, boating and live music once a week.
Tomorrow or soon, I am headed through North Carolina,and more or less in a northerly direction, me thinks. I'm used to sailing and going where the good breezes are. Well meaning non-RV friends seem to think I should be better organized and nail my travels down and plan every minute, my sailing friends seem to think if I turn up where I am headed in the next week or month or year, I am still on schedule...