It is probably the same sites you have already accesed. I would ask have you tried a patent search? I believe it is still free from the patent office but you do need the original patent no. Sowter makes a copy; you probably know all this so excuse me if I am redundant. There was a article in Sound Practices that told of winding your own and I believe they used an Acro. I'll check unless you already have. Sorry could not be more help. J.R.