The best bet is ordering online, although the selection still seems limited. Even at the expos I don't remember seeing any "Run Disney" merchandise. All of it is pretty specific to whatever race is going on.
Sometime the Team Mickey store in DTD has Run Disney Merchandise. Not lots of clothing, but other merchandise ( I got a license plate for my car there). The last time I was there for the Disney Princess Weekend, I had asked the sales associate why they didn't have any Run Disney Princess Half Marathon merchandise there. Her response was "They would be getting the Princess Half merchandise in next week". This really baffled me because the Race weekend was now! In fact, the race had just happend that morning. Anyone who ran the Princess and wanted merchandise would probably not be sticking around till next week. I feel like that was poor marketing strategy on their part. So, if anyone visited DTD the week AFTER the Princes Half, I am curious to know if they ever did get the Princess Half Merchandise in stock?
( Sorry such a long rant)