We went through a Barney phase too. Clothing is hard to find. We got the iron on transfers and made our own t-shirt for him. He loved it and wore it forever.
Depends which type of printer you have at home. You should be able to get the HP iron on transfers at any computer store and even at some of the bigger stationary stores.
I've bought white t-shirts from the street markets. But there is a really nice t-shirt lady in the lanes in central. She's in the 2nd lane. (Li Yuen St West) . We were there today and got a couple of Transformer Tshirts for D. She did tell me if we bring any image on a CD she can print the t-shirt for us. She charges around $50/ for custom printing.