If you use the custom search at the top of this page you'll find many threads for "C41 Stop Bath" you can even specify just threads with input from Photo Engineer, but in short, a standard acetic acid stop bath or indicator stop bath (made with acetic acid not an odorless stop bath made with citric acid) will work, just make sure it's a fresh bath and not one shared with your black and white processing. (I'd probably use my stop bath "one shot" in this application).
Since it is 'one shot' I use an extremely dilute 'normal ' stop bath: 5mL glacial acetic acid in water to make one liter. Other equivalences are: 6mL Kodak Indicator Stop Bath, or 18mL 28% acetic acid, or (yes, the same ultimate strength!) 100mL of (grocery store) white vinegar (usually 5% from grain, hops). I NEVER have problems. - David Lyga