I recently purchased a foreclosed home and found that the upper access panel cover was missing from the furnace. Does anyone know where I can buy a replacement?

Don`t know where you live.

But you might find a Goodman dealer give him your model number and maybe the serial numbers off your furnace and he can see if he could order you one

Find a good heating service company and check their warehouse. They sometimes stock extra parts and have old units there to scrap. You may find another brand will fit. I have a very large company near me and they have 50 or so sitting in the yard.