Firstly, contact the artist's management (record company) and negotiate with them. I have first hand experience of this, and I can tell you it is a very expensive process. Band merchandise is a very lucrative business. To buy exclusive rights to band merchandise, will cost you thosands, if not millions of dollars.
Secondly, don't be tempted to print pirate (unlicensed) merchandise on a large scale, as you will be subjected to expensive lawsuits and fined a huge amount of money, if you are caught.
Thirdly, you can buy official merchandise from official outlets, avoiding any copyright problems. My recommendation to you is, to go through those channels.
Printing and selling (or perhaps not being able to sell) is an expensive and very, very time consuming pastime. A six coloured separation silk screened t-shirt will give you an enormous headache, if you don,t have much experience of printing or have the right machines for the process.
Sorry to sound negative, but I am offering you sound advice, from my own first hand experience in this matter.
Good luck with your business!!!