Ya, many of the ones that you see are crosses. The ones that I know of, are the ones from Hubbard. They import their hatching eggs all the time from their HQ somewhere over in Europe. It looks to me that they breed a hen (heavy red broiler) over a heavy red Turken Male.
So if I was you... get yourself some red broilers or some of JM's birds... keep the females (maybe 4-5) and butcher the males. Try to locate a red Turken Male. Most of the hatcheries haver them and if you look hard someone on BYC may have some extras. Take the male red turken and breed them over the red JM hens... and there you go... you have a hybrid bird with heterosis and the necked neck gene.
If you wanted to, you can breed the hens from the F1 cross and breed them back to the Male Turken. This will give you kinda of a pure line, over time you can develop your own line. But, the biggest things with meat birds that I have found, that no matter what you choose, you want to do crosses. Either you cross two DP's or whatever... I've learned that they heterosis (hybrid vigor) is how the good results are made. If you look at any commercial cross.... either slow growing or the fast cornish x's... breeders have taken advantage of the benefits of the heterosis... and simply crossing two purebreds will result in the offspring growing better and laying more eggs as a result. From what I have seen, they always out preform their parents. I'm curious to see if this is the same with the cornish x's that I have but I doubt it, they may be the exception.
I don't think, that having a pure line is really important anyways... I think it's more about function over form.