Do you mean a door like this? They can be bought at quadratec or 4wd.com, I made mine myself, but I would have to say that they aren't the easiest thing in the world to make without the proper tools. I have well over 100 hours into a pair of doors, but only about $50 in steel.
You might want to just buy a set, unless you are a very capable welder and fabricator.
'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...