I suggest you to make a transaction first like converting your bitcoin to cash so you can shop anywhere without worrying if your bitcoin will be accepted. But maybe there are some other way for what you are looking for.
Also you can find someone or a t-shirt producer or designer that's willing to accept bitcoin. I have seen people who are offering this type of transaction for bitcoins > t-shirt and with their designed logo of bitcoin on it. Their sales are boosting because of the craze about crypto.