Try contacting Joe Hepsworth at http://www.jcunard.com/ .
 
He specialises in long range muzzle loading and BPCR. If he does not stock what you want he may be able to advise on US sources, or procure what you want.
 
Joe was the US team captain to the World Long Range Muzzle Loading Championships in the UK in 2005 and I think may be the captain for next years championships in South Africa.
 
You should also look at my Long Range Muzzle Loader web site at http://www.lrml.org which has historical and technical information on Whitworth and 'small-bore' match rifles.
 
There is also an associated internet discussion list at: http://sports.groups.yahoo.com/group/lrml/
 
David

You will get just as good results with cylindrical bullets.  The only advantage with hexagonals lies in using hard lead and paper patches, and then not as you would notice.