You can get Glow Dust in Elder Scrolls IV: Oblivion Remastered primarily in two ways: 1. Buying Glow Dust:

• Visit merchants in the Imperial City Market District. Two reliable vendors are Ogier Georick at The Main Ingredient and Claudette Perrick at The Gilded Carafe. Another vendor is Calindil at the Mystic Emporium, but you need a higher Mercantile skill to buy from him.
• These shops restock Glow Dust every 2-3 in-game days, so you can return to buy more as needed.
• Glow Dust costs around 50 to 100 gold per unit depending on the vendor

2. Collecting Glow Dust from Will-o-the-Wisps:

• Will-o-the-Wisps are ghostly enemies that drop Glow Dust upon death with nearly 100% chance.
• Common locations include:
  • Near Azura’s Shrine northeast of Cheydinhal, especially during the Azura quest.
  • Swamps and forests near water bodies, such as south of Imperial City along the Niben River or near Leyawiin’s Blackwood region.
  • Around some Oblivion Gates like those near Bruma.
  • Monster dungeons marked on the map often contain Will-o-the-Wisps.
• Will-o-the-Wisps are resistant to physical attacks and require magical damage or enchanted/silver/Daedric weapons or shock/fire spells to kill them effectively.
• They tend to appear more frequently at character level 10 and above

In summary, the fastest and easiest way is to buy Glow Dust from the Imperial City Market District vendors, but if you prefer farming, hunting Will-o-the- Wisps in the above locations is a reliable method.