An ISWC is a unique identifier for a musical work.
Every song has an ISWC.
Whereas an ISRC is a unique identifier for sound recordings.
More info on ISWC: https://www.iswc.org
More info on ISRC: http://isrc.ifpi.org
Where to get them
Your distributor will assign an ISRC.
But ISWCs will be assigned by your PRO.
If you use a publishing admin service, they can affiliate with PROs and get those for you.