Last year was the first year we attended MNSSHP. They did sell commerative t-shirts. We bought ours at the souvenir wagon outside the Haunted Mansion. It was purple tie-dye with Disney characters on it. It had the year on it too.
Only bad thing - DH was the one that bought it (we only got one) and he got the wrong size! It's too small for him ... a little too snug for me ... and too big for DD!