I remember some mini boss in a DLC drops one, or has a chance too. Doc Mercy just isn't dropping it for me.
5 Answers
Any legendary has a 1/20 (5%) drop rate from the boss it's assigned to, the infinity pistol is assigned to Doc Mercy. Any legendary has a 1/1000 (.1%) to be found in a chest, random drop, or in a skag/stalker/whatever pile.
The Gold Golem in DLC 4 (Tiny Tina's Assault on Dragon's Keep) has the Infinity pistol in its loot pool.
In addition to what Slowpoke said before me, you can also get any legendary weapon from legendary loot midgets. It is almost easier to get it from them, although they drop any legendary and pearls. Happy hunting.
I got mine in a chest in Sir Hammerlock. Ledgendary weapons appear in them more often for me than as drops. Just keep looking.
I got mine from a regular enemy, after thousands of hours of playing. It seems to be more likely in ultimate vault Hunter mode.
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While this is technically an answer, most players aren't willing to do months of grinding to get an item they want. It usually gets tedious in the first hour.– NissaApr 7, 2018 at 23:00