I always buy air filters for my central heating and air conditioning system unit on the internet.
The reason I do this is because you can buy them in bulk and get one heck of a discount on them this way.
Most of the time when I order the air filters for my central heating and air conditioning unit they arrive on time when the website tells me they will be arriving. However the last time I bought air filters for my central heating and air conditioning system unit they were late arriving. As a result I was also late in changing the air filter of my central heating and air conditioning system unit. I was completely out of air filters because I ordered later than usual, and just my luck the air filters that I ordered for my central heating and air conditioning system unit were also late in arrival! They were late by almost a week. So that was 1 week that I had a used up air filter in my central heating and air conditioning system unit. Thankfully it did not do any damage to the central heat and a/c unit or the indoor air quality of my home. If I had let that air filter sit unchanged for another week or so, there may have been the beginning of some serious problems with the air quality, followed by my central heating and air conditioning system unit starting to overwork itself. That could have led to a serious heating and cooling system breakdown and cost me money in heat and a/c repairs. I did not want that!