Almost any big box store, lumberyard or door company can order what you need.
There are a few catches with this which revolve around redoing 6' worth of walls which includes plumbing, electrical, drywall, etc...
Based on frequency of use I generally go with a solid core doorand increase the thickness of the cavity allowing 2x4's placed sideways to be used instead of those flimsy 1x's which also helps eliminate the dreaded drywall screw into the door. (Frame wall with 2x6's up to that point)
No matter if you go with a solid or hollow, make sure you use the best and most durable hardware available - Johnson Hardware comes quickly to mind as one good option.