Why can't I find the right size air filter?

I wish all air filters were the same size and shape. Wouldn’t it be fantastic if all the heat pumps, air conditioners and furnaces used a standard 20x20x1 air filter? It would be a lot easier to shop for them. Every time I leave the house for an air filter, I seem to forget the list and the size. I can’t remember the size, and I usually end up getting the wrong thing. My partner thought he would solve this problem by ordering a big box of air filters from the website online. After careful examination, we found out that our air filters are a strange size that only speciality companies stock. Well, we had some trouble finding them online too. My partner ended up buying the air filters from an HVAC parts warehouse online. Since it was a special size, they were almost 15 dollars each. We only purchased one, and the total bill was almost $25 with shipping fees. Even with one shipping charge, that’s a lot of money to spend each month. I don’t know what to do about that problem. We just moved into this house six months ago, and we want the air to smell fresh and clean. We always changed the air filter in our old house monthly, and neither one of us want to stop. With two dogs and a cat, the indoor air can get dusty quickly. We already have trouble with pet hair on all of the surfaces, so the air filter is a necessity. I guess we can order them month by month. Maybe we will eventually find them cheaper someplace else.
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