UK Solar System Data Centre
Copyright WDC C1 for STP at RAL.
The Kp index data runs from 1932 to the present day. Data from 1932 through 1936 is less accurate.
Final values can be found in the Solar Magnetic Activity (SMA) database, usually 1-2 months after the event. Daily provisional Kp values can be found in the Kp/Ap Provisional database.