Look at the travel agents or charities listed on the event's page on rundisney.com.
For this year's, I got hubby and son into the 5K using Acclaim Travel (
http://patstonetravel.com/walt-disney-world-marathon.php) They are listed on the rundisney page. I am unaffiliated with them. But I used them for this year's 5K and it was perfect.
The one difference between previous years and this year is that it's more of a package deal now.