It really depends on which machine you have. When I was unsure about which replacement filter I needed to buy, I gave the company that made my furnace a call. They asked a few questions about the furnace and then gave me the full specifications of the furnace filter that I needed. They also offered to sell me a replacement, but I felt the price was a little high so thought I’d shop online.
In the end, I found an online filter supplier for HVAC machines that was stocking the type of filter I needed and bought from them. It was delivered to my house a few days later. Then with the help of my neighbour who had some experience working with HVAC machines, I replaced the filter. Once I had the details I needed from the supplier of my furnace, it was really very easy to find a replacement filter.