If it really bothers you you could put the reference that you have first followed by something like 'original text - then the reference' in brackets or not. Or you could try find another author who has said the same thing and get their txt
But to be honest as long as you are quoting it from someone that has used and described it, it should be fine, although primary authors are preferable -
At the end of the day if you can't get the ref you can't get it (Oh have you tried google? sometimes you get luck and find it randomly on google unrestricted)
I asked one of the Professors at our Uni if it was a lot harder to write up back in his day and he said no! its a lot harder now because you have too much access, you have to consider everything. back then if they didn't see it, it din't exist!
Just a random ramble
S