okay, as a professional Catholic, I can give you some good ideas.
yes, nun names often have "Mary" at the front of them. Classic examples: Sr. Mary Clare; Sr. Mary Frances. You could go for the exotic: Sr. Polycarp, Sr. Euphrasia, Sr. Theodosia. The male names often signalled a particularly stern nun--Sr. Edward, Sr. Thomas, Sr. Francis Borgia. If you want movie nun names, there's always Sr. Bertrille (the flying nun), Sr. Luke (from the Nun's Story), Sr. Luc-Gabrielle (the singing nun), Sr. Benedict (from Going My Way).
If you don't care for any of these, I'm sure I can come up with more!
M60 H52 D20 M14 yrs OW-old gf from 1986 bomb-5/18/08 H filed for D-9/10/08 D final 4/24/09 xH remarried (not OW) 2012