It should be in $ORACLE_HOME/lib/, and this path should be in
LD_LIBRARY_PATH.
If you don't have it or the file is corrupted, try re-installing Oracle,
maybe somewhere else, and copy the file from there. I've heard of
installations where a second attempt installed the correct file, but at
first there was only a corrupted or zero length file.
to find the file, try
find $ORACLE_BASE -name "libnldapj8*" -exec ls -l {} \;