Kernels for currently active missions where NAIF produces the kernels or otherwise has access to them may be obtained from the NAIF server using the links above.
Also available from these links are kernels for some special cases: missions where an archive has not yet been initiated or has not been completed (e.g. Galileo), missions that preceded the existence of the SPICE system (e.g. Voyager, Pioneers 10 and 11) and some missions that were not planetary missions (e.g. Genesis, Hubble Space Telescope, SIRTF). The kinds and amounts of SPICE kernels for these special case missions varies substantially, and in some cases the provenance and/or quality of the source data used to make the SPICE kernels is unknown.
Additionally, you may access the complete collection of available kernels from this link.
In most cases of still active missions, SPICE data from early parts of the mission have already been archived at the NAIF Node. ("Early" usually means up to about 6 or 9 months before today, but this can vary.) If your research interests do not require the very latest data from the mission, you will find it much easier to acquire and work with the archived SPICE data available for your mission of interest, available from the PDS SPICE Archives portion of this website. Be sure to look at the START TIME and STOP TIME for the data set of interest to see if the archive will cover the times of interest to you.
Please note that kernels produced by agencies other than JPL are usually available only at those agencies, and may not be
available to other than the flight project's team members. (By
agreement between ESA and NASA, and JAXA and NASA, kernels for a few foreign missions are mirrored at NAIF for the convenience of U.S. participating scientists.)