I've had the best luck with horns bought from October Country..they have 12"+ ones, unpolished, for about $5 each...I buy 5 or so at a time, and the price and shipping both drop. Their horns are from South Africa, and make really good powder horns...
As a kid, I walked the length of Newark NJ to get to a slaughter house to get a horn..they gave me a skull with two horns still attached...on the walk back, I got a lot of stares! I would be surprised if they'd do it today, but if you can find a local slaughter, that'd be a good source. Around here, people de-horn cattle before the horns are worth working with...Hank