I believe that particular yarn has been discontinued.
Have you tried seeing if you can find a skein for sale or trade by someone online? You could try ebay or Ravelry ... several Ravelry members have this yarn stashed, although none are listed for sale. If you really want to finish the pattern as written then you could always try contacting them to see if anyone would be willing to part with a skein. You didn't mention what color you were looking for, so I'm not sure whether this would be an option or not.
Otherwise, Jan's suggestion is probably the simplest if you can make it work with your pattern. With something like a blanket no one but you will notice that it was supposed to be a few rows longer!