The height of the doors has nothing to do with the size of the header. The width of the rough opening and the overhead load determine the size of the header. All headers aren at least double thichness of 2 by lumber. Plus a half inch spacer. So, the width of the RO for the doors will dictate whether 2"x8" or 2"x10" are necessary.

Either way, the header will add 8+" minimum from the ceiling, so to have 5' tall doors, you will need to start with a ceiling height of about 6' tall.

As far as the doors themselves, you might be able to order them at a real mlumberyard---not a big box. Or find a cabinet shop that would make some. Or fond a woodworker to make some.

Thanks for your help--you've given me food for thought.

Are the walls you are putting the doors in even weight bearing?

If they are NOT weight bearing a header is not required beyond something to fasten the door jamb onto.