For Desolator we need following:
Fervor - another faction blueprint
Zeal - you can get it from Hyram in Steelcap district outpost, he sells
transcendent relics, just have to “farm” him a bit
Ugdenbog Sparkthrower - regularly drops from Ugdenbog golems or can be bought from the plant vendor in Ancient Groove
Silvercore Bolts - another faction blueprint
All other ingredients are incredibly easy to get and ull get plenty of them by just playing the game (some rare crafting materials and regular materials).
Lets not pretend we are crafting Ignaffar Combustion here 
Considering how powerful this rifle is, its incredibly cheap to craft.