SS --- Quality superplasticizer can be very difficult to come by. Start by trying to find a cement/concrete specialty supplier in your area. The big box stores probably won't even know what you are talking about. I use a product called Melment F-10. It's manufactured in Marietta, Georgia by SKW Chemicals for DeGussa. It is probably the best & most consistent high-range water reducer made. Good stuff. Unfortunately, they only sell it in 55# bags, but may be able to direct you to a local source who sells smaller quantities.

Best of luck --- Tango