Cleanstream Pump Action Enema Bottle with Flexible Nozzle and SyringeStyle Handle, 300 mL Capacity, Silicone Material, Easy SelfCleansing Douche System for Controlled Personal Care

I personally caution against these for two reasons: (1) putting pressurized water up there is unsafe, and (2) flow rates and temperatures can be unpredictable. Also I prefer to keep my filtered shower head anyways. Instead I recommend this thing (much cheaper), it has volumetric markers, is not pressurized, and you have more control of the temperature.