In NYC you can get this at the Health Food Store on 125th Street and Lenox Avenue (I believe). It is across the street from the corner where there is a McDOnalds, Starbucks and Cingular right next to eachother. (Sorry but I forget the name, and never paid attention to the exact street).
I use the Smooth N Shine leave in conditioner and just put the castor oil and water into it and mix it up into a spray bottle. I spray this mix onto my hair before I rollerset. The castor oil is sticky so be careful when combing your hair with it in there, or just add more water to the mixture.