There are charity groupos. I believe most of the groups are sold out for Dumbo Double Dare for 2013. They may have Half Marathon slots available. Check out the runDisney web site. There is information about charity groups on the Disneyland Half Marathon page.
On the RunDisney site you can sign up to be notified when registration is going to open. There's also usually an AP/Disney Visa card pre-sale, which they mention on the RunDisney and AP Facebook pages.
They should still have some spots with charities, as mentioned. The other way in at this point might be through tour operators, who also usually have spots available long after the race sells out. They should have those listed on the RunDisney site.
Look at charities. I had to do that one year when I signed up 3 weeks prior and it cost me $700 to run. Didn't think I was going to be able to make it that year and my schedule opened up at the last minute.