I have an exterior double door that needs to be replaced.  Measurements of the opening are 48x82.  Right now it has plain steel panel doors, but they are damaged.  These doors go to a storage area, so they have no glass.

Home Depot quoted me over $500 for new doors, no installation.  

Have any idea of who makes an off the shelf replacement door for this opening?

48x82 is not a standard size that you're going to find off the shelf at any box store.  Most major companies that sell exterior doors can do that size, but a 48" frame width double panel is likely the narrowest that they will go, is likely going to be a "standard" size for them, but it will not be one that is available as on hand inventory, will have to be ordered which raises the price vs a "stock" item. To keep the price down you can do a wood exterior frame(instead of aluminum clad), but based on that price i'm guessing that's what you're doing already.  

Only other suggestion would be to go to a place like Habitat for Humanity ReStore or a similiar architectural salvage yard and see if you can find a door panel that is the right size you could use, if you can find one without hinge routes or lock prep and do that yourself it would make installing it easier as you could prep it to match the frame.

Other option depending on your skill level and the exterior material(siding, stucco etc) is to reframe for a standard size product of 36x82 and shrink the opening to a standard single door size which will cut the price of the door, but require lots more work to frame then deal with the now smaller opening and making the exterior and interior look right.

pics?

I've got to keep the opening at least 45".  My tractor is 44" wide  The current doors are double doors which give me the width.

If I can't find double/french doors that will work I might as well hang a garage door that looks like a door.  Price seems like it will be similar because I won't be using an actual opener.

Don't have a picture handy.  

It is a double door in a framed opening.  Threshold at the bottom.  Doors are a pre-hung unit.  Exterior is brick veneer.

Can you reframe the opening to 48" X 80"?  You might just add fixed "blocking" at the top to drop the opening 2".  

That would allow using two standard 24" X 80" doors.

Order a custom size Therma Tru door. Best bang for your buck. Most of my customers end up choosing a therma tru door as they are made very well.