If you are doing flyers, do not discount your Student Newspaper either. You could do kind of an announcement and ask the Student Newspaper to print it for you.
(I know that Student Newspaper can be a bit...err...."leftist"sometimes, but if you are just doing a general announcement, they can't really crow about that). Have some contact information in there, and hopefully some of the disappointed rushees will see it in the paper. Try to get it to run more than once. Or you can have a Letter to the Editor, just letting people know that you are wanting to contact women who are interested in bringing a new NPC sorority to UCSC and have your email address given there....that shouldn't offend anyone.
To be honest, I don't know how cooperative Gamma Phi Beta will be in providing you with contact information of girls who were not extended bids.......with the 10 that you do have, see if they know of any others.
If any of the rushees has unusual names, you could try to track them down through the
UCSC Email Directory (you can use first name or last name, or both):
Also, does UCSC have residence? Don't forget to advertise (when the time is right) around the res halls too