I am looking for the filter on our furnace and cannot seem to find one anywhere. Our furnace is an icg ultimate iii. Unfortunately I'm unclear of its age but is probably in the 20 yr old range. Ty for your time

Maybe it doesn`t have one, how long have you lived there?
Maybe someone took it out and didn`t replace it
Sometimes filters are behind the return air vent,
Describe the set up of the furnace, is it upflow ? Downflow? In the attic? In basement?
Have you looked in side the area of where the blower that blows air into the living area,