The original material was of course the now scarce asnd infamous Chadwick's no. 477 darning wool, which has become so ridiculously ecpensive that it's hard to believe.
But you don't need it to tie the fly. As I also mentioned in one of my Killer Bug-FPs, it went out of production even in Sawyer's lifetime, and he began using an alternative. I then tested the (excellent) substitute by Semplerfli.
The Grunde Løvoll from Norway contacted my and informed that Jamieson's of Shetland also made a nice wool yarn, which was also a good substitute. I went on their website and shopped around, and got a bit carried away. I bought five different yarns, two of them closer to the Chadwick's no. 477 than the others. These are the ones Grunde sent me some samples of, which I use here. One is lighter in colour than the other. The darker one os a good match to the original, but it has some blueish/greenish fibres in it that the original hasn't got, and it's a bit more pink. None the less, good alternatives.
I came across a few other colours that I found interesting. An orange one, a more hot pink one and a nice "dark cinnamon brown" one. Would a trout or grayling ever reject one for the other? We'll never know by I'll be tying variants with all of the yarns, including the Semplerfli one, over the winter, ready for next season.
I can't promise it, but (I hope?) this is the it for the Killer Bug for a while. On the subject of it and Frank Sawyer, if you haven't already, I recommend "Keeper of the Stream" and "Nymphs and the Trout" for you to read this winter. Both are excellent reads.
Have a great weekend!