You can read the Demon Slayer manga online at these online websites. You can also purchase the volumes you need on these websites.
The first volume of the Demon Slayer manga is free to download - Polygon
Kimetsu No Yaiba (Demon Slayer) Vol 1 Flower of Happiness – Moko-Chan's Translations
Demon Slayer: Kimetsu no Yaiba Chapter 130
VIZ Read Demon Slayer: Kimetsu no Yaiba Manga Free - Official Shonen Jump From Japan
VIZ Read Demon Slayer: Kimetsu no Yaiba Manga Free - Official Shonen Jump From Japan
23 Must-Read Modern Shonen Manga
The 10 Best Manga Volumes Of Demon Slayer (According To Goodreads)
Demon Slayer: Kimetsu No Yaiba” Vol. 1 & 2 – Multiversity Comics
Demon Slayer: Kimetsu no Yaiba, Vol. 5 by Koyoharu Gotouge, Paperback