I can't seem to find a vented metal door for my furnace room. The two vents at the top and bottom of the door are all rusted out and the door is in pretty bad shape too. Are these doors custom made or is there a place where I can get a new one?
You have a vented exterior door? This is unusual -- most exterior doors are designed to block air and water, not to provide ventilation. Yours was likely a custom modification. Not to be confused with interior doors, which are more commonly vented in order to help distribute air flow between rooms where the doors are frequently closed (e.g. bedrooms).
Depending on your ventilation requirements, you may also be able to replace your door vents with other vents, such as ones installed in your wall. This might provide a longer-lasting solution, as any vent cut into an exterior door will expose that door to moisture and could lead to premature failure. However, if you make any changes to the room's ventilation, you must make sure you are maintaining adequate ventilation for your appliances. If not adequately vented, you could risk introducing dangerous combustion gases into your living area.
My guess is that in most basements, whoever built it just took a standard interior door, cut a hole in it and screwed in some vents on top.
You can definitely buy doors with these built in. As to where to buy, this is too localized to answer on this site. Call some local lumber yards, home improvement stores, doors & windows shops to see if anyone can help. They might have stock, or they might build one custom, or maybe they will just modify an existing door for you.