I have a Johnson furnace, model C24US-A. I just moved into this house and am looking to replace its filter but cannot find it. Where is the filter located?

Open the blower compartment door & look on both sides of the blower. There is often a track in which the filter slides.

Some don't have a filter on the air handler, they are in central wall mounted returns.

Thanks. I looked first for a return in the wall. Finding none I looked for a slot for the filter on the furnace. The only thing that looks likely is a ledge on both sides above the blower. Do you think that a potential spot for the filter? There was no filter present.

Yes, that is most likely where you will have to place a filter/s. Or you may be able to purchase correctly sized filter grills and place them where the current wall return grills are.