The talk in Hogsmeade is that online store Hot Topic has available for purchase a black t-shirt featuring the infamous Sirius Black wanted poster. The cost is $18 USD plus shipping.
Availability Update 1/19/04: Hot Topic told HPANA today the shirt will be available to purchase online again around the second week of Feburary. The online store currently shows the t-shirt as "completely sold out".
Also, while not for sale, here's a look at what the film crew of Harry Potter and the Prisoner of Azkaban wore while on the job, courtesy DigiCasey.