Currently, as of this writing, a partial collection (Seasons 1-5) is active on Archive.org under the user "FloridaManArchives." However, given the volatile nature of copyright enforcement, that link may be dead by the time you read this.
Archival Status of SpongeBob SquarePants Episodes on Internet Archive (archive.org)
For a more stable (and legal) experience, you can find the episodes on: : The primary home for all seasons.
3. Preservation or Piracy? The Legal Battle of Bikini Bottom