You only need to be a SoCal resident to get the SoCal and SoCal Select passes at DL. Also check with your local AAA they have some better than average deals on tickets, may have some on the passes also.
6 Flags may occasionally run a buy a one day ticket get an annual pass promotion. Those run in the slow times of the year though.
BTW, if you get passes at DL, and decide to get parking, get it on the person who will go most often. Getting it on Dad's pass when Mom and the kids are going to go most and Dad not many times, it doesn't help that much. The passholder with Parking needs to be in the car to use that pass to park. It isn't that big a deal in the long run, but it really pisses off the folks when an AP trys to bs you when they didn't pay attention to the rules. DON'T LET YOUR KIDS LEND THEIR PASSES to anyone for any reason. We probably collect 6-10 a week for misuse. (I work in DL parking on a part-time basis) and that is a several hundred dollar lesson in following the rules.
And there ain't nuthin cheap at DL.