Have you tried an home improvement store? 8' is not that common for residential, but they might have at least one in stock and then you can order another one.
You can also try Probuild.
ProBuild Store Locations | ProBuild They should have more stock than a typical home improvement place and they sell to builders, but will usually do a one off for a homeowner. Surely they will have a 3'0x8'0 door laying around.
Next stop would be any commercial door company. That is a definitely a common size for a commercial door and they absolutely will have some laying around, I would bet my life on it, but you may have to take a solid core door.