I dont think BCB has authorized anyone to seel the T-shirts online ( Shame on them!!). You can buy the legal T-shirts from Dhaka. I believe Shah Sports or Intra sports of Elephant Rd. sells them legally. You can always try Ebay..
However, during the World Cup, they might sell the T-shirts online via Cricshop from Cricinfo. I bought mine from them for 70 dollars...