Where can I get better glyphs in The Witcher 3?
There are three levels of Glyph - Lesser Glyph (2%), Glyph (5%), and Greater Glyph (10%).
They can be found in loot, but this is random. The chance of finding better Glyphs increases as you gain experience.
You can also buy them from shopkeepers, armors and blacksmiths. Again what they have is random.
Finally you can find Diagrams for the different types of Glyph. With these Armors and Blacksmiths can upgrade Glyphs, but only if they have enough experience.