An alternate means for obtaining SPICE data is to use the PDS Central Catalog's
Keyword Search mechanism. This catalog provides access to archived SPICE data
for NASA and some foreign missions no matter at which PDS node they are archived.
Enter "spice" and any other relevant word(s) in the search field box and press the search button. For example, you might enter "spice MRO" or "cassini spice" or "SPICE MEX." Look under the "Data Sets and Information" heading. The search engine will return one or more hits pointing to archived SPICE data. (If a data set resides at a foreign archive, this will be indicated: for instance, it might say "(from ESA).")
If you enter "cassini spice" the catalog will be searched for all entries containing either cassini or spice—this is likely to be a very large number, although the data set of primary interest to you is likely to appear at or near the top of the list. You could shorten your search results a bit by using "and" in your query: "spice and cassini". Then look for a hit of the form
"Data Set: SPICE KERNELS V1.0" for a SPICE dataset hosted at the NAIF Node or
"Data Set: (mission_acronym)-(mission phases)-SPICE-6-V1.0 (from ESA)" for a SPICE dataset hosted at ESA.
A "spice" query will usually also return references to archived instrument data for the named mission since documentation for those data usually include references to SPICE.
Once you select a SPICE data set from the search results a new window will appear, offering a variety of information. Next to the "Search /Access Data" heading you will usually see three entries: NAIF Online Archives, NAIF Data Set Subsetter, and NAIF WebGeocalc. Selecting "NAIF Online Archives" will take you to a webpage from wich you can select individual components of the archive, which is usually not the best choice unless you specifically want individual files (kernels). Selecting "NAIF Data Set Subsetter" will allow you to specify a time range of interest and have a subset of the archive created for you that is appropriate for your selected time range. Selecting "NAIF WebGeocalc" will bring up the WebGeocalc geometry engine tool in your browser. If the data set is also available at another (foreign) location, an entry for that will also be shown.
You may also select the dataset name next to the "Dataset Information" heading. Picking this will display a very useful overview of the entire data set. (In PDS lingo it is the "dataset.cat" file.) If you are not already familiar with the mission's SPICE data, it will be worthwhile to read this.