I am sure that some questions are just asked because people like to take up space on Yahoo. This is one of those questions, I am sure of it.
You get official Harley shirts at the same place where you buy new Harleys-at a dealership. Don't be afraid to go there. They will not beat you up or rob you. They are a buisness. You buy things there.
You can also get some knock off type shirts at various places online. You mearly need to do a websearch.
