Bella,
Metropolis Graphics in Orlando does/did all of the UCF Greek and campus event shirts. You can price them out through Metropolis-- they will even design it for you.
Go to your chapter's social chairman and tell her your idea. Offer to price out the shirts and do the leg work. We love sisterhood shirts-- they don't have to be in honor of a date function or chapter event.

When I was in college we had one that said "The only thing worse than an ADPi without a man is a man without an ADPi."
Or, instead of proposing a sisterhood shirt to the social chair... you could approach your new member coordinator and recommend it as a shirt for the new member class... or as a shirt to celebrate when your class is initiated!
Or... you could request to meet with both your new member coord and the social chair together to share your idea... that way, you're not stepping on any toes and you have two officers to applaud your initiative!