They're incredibly hard to find! It's almost impossible to find one with The fourth Doctor's image on it. You might come across one or two designs on "Ebay" but that's it. I find that strange being that Baker's incarnation is the most popular and best(in my view at least). Can anyone help?
I have an excellent fourth Doctor T-shirt that my wife bought me last year. It's got a massive image of him from City of Death and the quotation "I deny this reality!" They're not too hard to find - I think Forbidden Planet, Sci Fi collector and maybe even Play.com/HMV have them. Pretty sure there's plenty on ebay too.
The only Dr Who related T shirt they offer at the Mall here is several sizes of the same image-a Tardis coming out of Vortex. I might not be able to fit into the sizes available, but it would be a nice collector's item.