The article is Joan Robinson, 'Economics is a serious subject: the apologia of an economist to the mathematician, the scientist and the plain man' (1932). It's footnote 1 in Coase's 'The nature of the firm,' which I was prompted to read in more depth by a recent HN comment, so it's not an especially obscure document! The university library I spoke to does do inter-library loans, but I need to find a local library that will take one out for me. I haven't got around to doing that since I moved states 18 months ago, and I'm no longer affiliated with a local university. It's a pain, but something I probably would have resolved sooner if there weren't so many books digitised in the various proprietary databases!