From Mohammad Sakib Arifin's answer to Why are women not allowed to go out alone even in today's society?:
Shia view: According to mainstream Shia scholars, it is permissible for a woman to travel alone.
Based on the author's track record on this site, I suspect it's a correct claim, but...
Question: Which mainstream Shia scholars say it's permissible for a woman to travel alone?
Googling revealed inconsistent posts on ShiaChat, and I'm not sure how much I can trust them. Googling
site:sistani.org woman travel and
site:leader.ir woman travel didn't reveal any easy, general answers.