R' Alfasi's laws of Niddah are actually hidden in the second chapter of tractate Shevuos. If you don't have a printed tractate handy, then see pages 767 through 778 of hebrewbooks.org's Sanhedrin-Makkos-Shevuos PDF.
As a courtesy to the reader ש”ס נהרדעא also includes a second copy of R' Alfasi's laws of Niddah inside tractate Niddah. This second copy is not typeset as nicely. You can find it on pages 283 through 294 of hebrewbooks.org's PDF.