Call theaters in your area and ask for referrals to their costumers.
Some theaters with in-house costumers even rent out their costumes once a year or so (or sell their outdated or too-worn-for-show-use costumes).
You may very well be able to locate a nun's outfit that way, especially if you run across a theater which has produced either 'The Sound of Music' or 'Nunsense.'
Good luck!
P.S. Failing that, you could always ask a tailor or seamstress to create a suitable habit. As long as you're willing to pay for it, they should be able to produce it.
