To get thunderbug juice, you simply need to kill thunderbugs. You can find these in G-rank snowy Mts., Swamp, jungle, and the Tower in area 4(G-rank is any guild quests HR 4 or above). Just kill the thunderbugs(which are like floating balls of light), and if they drop a shiny, pick it up and you might get thunderbug juice. But be careful: when the thunderbug turns red, it will attack you. This has a chance to paralyze you. If you have any further questions, please feel free to email me at Monsterhunter.firstname.lastname@example.org Enjoy.
I've found that using a poison weapon they drop shinies much more often.