Go on, you never buy me anything.
Usually I sneer at people who buy their t-shirts on Internet, but for once I have found an in-joke so in that I want to wear it. For the baffled, here is a partial explanation; for callow youths, here is the other part.
And can someone explain to me what is funny about a More Cowbell t-shirt? It is not the sketch; at best it is a reminder of the sketch. Is it meant to tell the wearer's viewers that the wearer has a great sense of humour? If so, it doesn't work: it says the wearer is trying too hard.