Read OTW's suggested threads!
Seek out sorority alumnae you already know to write recs. It's a bit late in the game to contact complete strangers, in my opinion.
As far as waiting for informal recruitment, know that not all chapters hold COB events. Those who do will do so pretty much immediately after formal recruitment is over. If that is the route you wish to take (though I'd urge you to go through formal recruitment), you can get information from the
Office of Fraternity and Sorority Life once school starts about which chapters are COBing.
Go Knights!