The most diverse collection of rock t-shirts is most probably available on eBay as these will include tour t-shirts from all over the world. Alternatively one can purchase t-shirts on Amazon, Rockabilia and Grindstore websites.
There is nothing stopping you from doing so, but it would just go rock hard. You can buy t shirt paint or t shirt transfers which can be printed out and ironed on. You could use outdoor paint if it is for a one off.
The people will have to firstly find your t-shirts or shirts. After that, they will have to click on it and press the green 'Buy Now' button. Once they follow the prompt given, they will have bought your t-shirt or shirt.
One can buy Quicksilver shirts at department stores like Macy's. At the same time, one can buy these shirts online when one goes to the sites of quicksilver dot com and zappo dot com, surfstitch dot com, etc.