This is a guide to obtain all 8 bomb diagrams in Witcher 3: Wild Hunt (Dancing Star, Devil’s Puffball, Dimeritium Bomb, Dragon’s Dream, Grapeshot, Samum, Moon Dust, Northern Wind). This will also net you the Bombardier trophy & achievement.
The earliest possible moment you can obtain all the bomb diagrams is after you visited the Royal Palace of Vizima. You will then gain access to rest of the map. The herbalist that sell all diagrams are north-east of Oxenfurt. It shall be noted that you can not enter Oxenfurt yet as you need a passport, but you can swim around the town still reach the dwarf herbalist. Refer to the video guide if you want see the easiest possible path to the herbalist.
Each bomb diagrams will set you back 250 crowns and a total of 2000 crowns to obtain them all.