The Magic Missle and Rogue COM both require the Tiny Tina DLC. Your best bet for Magic Missle is Badass Wizards and you can get the Rogue COM from treasure chests in that DLC. For the Transfusion Grenade (and perhaps Stockpile Relic), your best bet is probably to farm the How Marcus Saved Mercenary Day DLC. The loot train gives 4 grenade mods and a few relics each time you kill the Snowman.
You could also try farming LLM in WEP (for a Stockpile Relic) as long as Doctor’s Orders is open and you haven’t picked up any (or at least not more than 1) of the Notes or farm LLMs in Thousand Cuts with Rocko’s Modern Strike open (don’t turn it in) for a Stockpile Relic.
As for the Droog and Snider, those can be gotten from almost anywhere (Bullymong / Skag piles, chests, vending machines, etc). You might want to consider using Shift codes to get Golden Keys. Most of the time you will get 2 Purple (or perhaps E-tech) items, unless you get either a Rocket Launcher (in which case you will get only 1 item) or Relics which are almost always Blue.
NOTE: The only place I have reliably gotten Transfusion Grenades from is the Loot Train…
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