Could be a hard one to find. I bought some a year or two ago, but it was not an easy undertaking. Had to buy them through an auction.
At this point, I believe the latest crop failed, and so they're not readily available.. Only major supplier I could find so far that even has them listed is Reimer's but they list them as out of stock:
http://www.reimerseeds.com/burpee-big-girl-tomato.aspx
There's a link on that page to have them notify you when they're back in stock.