You didn't say if the door is an entry door, close door, bi-fold, French, pocket, metal, hollow-core, or the like.
You can start by shopping at your local Home Center. From there, you'll see the type of door you need, and why 1 costs more (or less) than another. If you DON'T see the door you like/need, stop at the Building Materials Desk to look at cataloges.
My best to ya and hope this helps.
Jay J -Moderator
PS: BEFORE you buy, ask about what type of 'maintenance' the door requires. This way, you'll be sure it's one you'll be able to 'live' with ...