Snort Prelude output should provide an implicit classification.reference entry for SIDs less than 100000 (higher than this are likely to be Bleedingsnort or custom rulesets).
For an example rule 1742, this URL would look like this:

http://www.snort.org/pub-bin/sigs.cgi?sid=1742

Form an example resulting IDMEF classification.reference like this:

classification.reference(0).origin=vendor-specific; \
 classification.reference(0).meaning=Snort SID; \
 classification.reference(0).name=1742; \
 classification.reference(0).url=http://www.snort.org/pub-bin/sigs.cgi?sid=1742; \