It was published in What's Your Story, a postcard book published by Waterstones. The book sold out almost immediately. There are some copies available from marketplace sellers on Amazon UK (http://www.amazon.co.uk/Whats-Your-Story-Postcard-Collection/dp/1902603575/ref=sr_1_1?ie=UTF8&s=books&qid=1225467348&sr=8-1).
MS, the postcard book of which profits went to charity was a collection of postcards, each containing an original piece of work from 21 famous authors and several chosen pieces from the public. They sold out within 2 days of release. Waterstones totally underestimated the pulling draw of JK Rowling. Of course they know she will draw attention but not to this scale. So a few weeks ago they printed more, and I was able to buy one 2nd time around having missed the rush 1st time. Give the stores a call to see if maybe they have any left because I dont think the 2nd time availability was shouted out loud for everyone to hear. JK wrote a scene involving James and Sirius from a non-existent prequel. Its a delightful piece and gives us a wonderful snapshot of James and Sirius in their heyday.
Ok, I just had a look at Waterstones website and dont even have the page for this book anymore. So I think its unlikely they will print anymore. I guess this is pretty muc a collector's item and as Renegado suggests, the marketplace is the only way to get it.
Try googling for something like Harry Potter Prequel, or variants thereof. It was on the Waterstone's What's your Story? website http://www.waterstoneswys.com/ for a while, and you might find fansites that kept a copy.