I am a critic of them being sold pre-race... and of owning one pre-race. However, they are sold at the expo and from what I know from my experience over the years... if you see it, grab it. There is no guarantee of finding it again 5 minutes later, much less pre-race.
Here is the rub. A limited selection of race gear is usually at the store at WWoS or at Team Disney (DTD) (though this year they did not send any to Team D) for about a week post race. After that, it is gone, only to be found in one of 2-3 local outlet stores in the Orlando area a few months later. Since the Sports merchandise and regular Disney stuff are not really handled under the same group, there is no way to pick up shirts, etc except in person. It's kind of sad as I think they miss out on selling stuff at retail for a few more days as opposed to dumping it later on teh outlet market.