Does anyone know where I can find a copy of 'The Sadeian Woman: An Exercise in Cultural Histroy'  for free by Angela Carter?
If your local library does not have a copy of The Sadeian Woman (which they should because it's a classic in it genre), the local library should be able to obtain it for you from another library on loan.
If you cannot obtain it through a library, getting it for free is problematic, but there are a couple of websites that have copies for relatively low prices (about $5-$8, which includes shipping). The first one listed contains the largest number of books:
It is also possible that the text is available either in full or part on an internet site, most likely from a university or a site dedicated to Angela Carter. If you see a reference to the text in pdf format, and the site has an .edu as the identifier, that might be your source. I suspect, however, that you will have to try a commercial source.
Hope that helps.