I don't know if you're still looking for a national to possibly affiliate with, but you might want to check out Kappa and Chi O because neither has a national philanthropy. My undergrad chapter worked with a domestic abuse shelter as well, and not having a national philanthropy was great because we got to pick something we really believed in that suited us. With Kappa, you'd get a lot of local support, too, there are LOTS of chapters in California. PM me if you want to know anything else about Kappa, I think the public section of the national web site isn't as informative as it could be. It's
http://www.kappakappagamma.org
Good luck!