They don't put them out until the actual party. Several gift shops had them (Star Traders was where I got mine.) I believe they are different each year.
We got our MNSSHP during the party at a kiosk stand on Main Street. They were purple tye died shirts and were pretty neat looking. As far as cost goes, if I remember correctly, the adult shirts were definitely less than $20 and the children's shirts were less than $15. I could be wrong, but I really don't remember them being more expensive than any other Disney T-shirt.