Check the Internet Archive first. If you find a legitimate PDF, treat it like the historical artifact it is. Do not repost it for profit, and consider donating to digital preservation funds. And if you do find the fabled Volume 2... let the rest of us know.
Private trackers dedicated to manga, such as Nyaa.si (for raw Japanese content) or Madokami (historical archives), occasionally seed a complete Bokko-chan PDF. Look for a file named [Complete] Bokko-chan (1969) [3 Vols] [HighRes].pdf . Be aware: these files are usually 300-500 MB due to the high-quality grayscale scans required to preserve the original gekiga ink bleed. bokko-chan pdf