The custom t-shirt in question is the one that bears the message of your choice; perhaps a message that indicates what you stand for. It could be a t-shirt you are looking to use in a particular event (so you get it to show it in mind). It could also just be a t-shirt you are looking to use as day to day clothes.
If you opt for this route, though, you should be prepared for the meeting and face a number of problems. One of them is the fact that the store is related to tailored clothes (and that includes t-shirts on the 'menu' they are very few and very far between indeed).
In other words, you should be prepared for the crowd quite a bit, if you get custom sublimation shirts online. And since this is a t-shirt that is the essence of human labor requires immediate application, you should be prepared to pay a price that is pretty neat for them.
Alternatives to go to a clothing store brick and mortar will be adjusted online stores that promise to help you 'design your own' custom t-shirts. This arrangement is that the online store will provide you with software programs through which you can determine what you want in terms of t-shirt: the desire of making software, one would say.
Once you determine what you want in terms of custom t-shirts, you leave it to people in the back of the store to make it up to you: exactly as you specify it. Typically, they will make a t-shirt for you, customize them for you, and then ship them to your door.