The link Thaiturkey posted for the UK site isn't accurate in the United States, as we still have an approved preventative drug that is available, called Nitarsone. However, it is a little hard to come by = sold in bulk only by prescription from what I can tell.
But, some feed manufacturers do sell medicated foods with this product. I bought Turkey Grower HS Medicated Pellets from Blue Seal Feeds. It is approved for use on both chickens and turkeys. You need to discontinue feeding 5 days before you butcher meat birds.
http://www.blueseal.com/livestock/poultry/feeds.php
Blue Seal is a chain of feed/farm supply stores kind of like
Tractor Supply from what I gather, and they have stores across the Northeast from Maine to Maryland and Pennsylvania. If you aren't in this region, they have an online store. They don't list this product in the online store, but if you e-mail customer service, the headquaters in Maine will be happy to ship this product to you. Alas, shipping costs more than the feed, which is only $10 a bag.
Your other option is to buy a bottle of the drug Fishzole aka Flagyl, an antimicrobial. If you see any signs of this disease, the first sign being yellow or green diarrhea from what I gather, you can treat with this drug. So, you might want to have this on hand just in case.
http://www.amazon.com/Fish-Zole-Metronidazole-250-Tabs/dp/B0002YFA0E
I have never had turkeys before this year, and I got a little freaked out in reading about blackhead since it sounds so nasty, but I guess it's not really a very common disease. Most people on here keep turkeys with chickens without any problem, and odds are, if your birds have been together for a while without any problems it's probably not in your flock or your ground.