We had an inside tip in February that shirts would soon be available at the Emporium on Main Street USA.
Anyway. I called the merchandise line a few times, and finally, they're in! We ordered one of each of the three designs sight unseen. But, I have to say, we're very pleased with the results.
I haven't seen any of these on Ebay yet, nor have I seen them on any of the websites that sell Walt Disney World merchandise. Hopefully, it'll be available soon through those venues since the shipping charge the merchandising hotline charges is ridiculously high (a per item fee, rather than per shipment).
At the time that we ordered the Haunted Mansion t-shirts (around June 17, 2008), they were just under $20 each, and shipping was around $6 each. The order qualified us for one of those special deals like they do in the parks, and so I got a wheeled, nylon duffel bag (just a big duffel bag, but it had the extending handle and wheels like luggage does) for $20- and that didn't have a shipping charge. It's a really nice duffel, too, and has a picture of Mickey Mouse, and it says Walt Disney World, 1971 on the side. I forgot to take a picture of it, though. Sorry.
If you want to order the t-shirts, or anything else from Walt Disney World, click the link to go to a page with contact information for the Walt Disney World Resort merchandise department.
They ship UPS, and ground shipping will take approximately 10-15 days due to the time it takes to actually get the merchandise to ship.
Update: All three t-shirts are made in the USA- so support our economy three ways! Production (where they're made), Distribution (by our favorite company ever), and shipping (UPS).