What where your methods to have it drop more?
There’s absolutely nothing you can do other than keep farming Doc for it. Yeah, it could pop up in a random chest, but it’s highly unlikely it will. Failing that, trading for it.
it could drop from the loot mignet too
Wouldn’t farm either loot midgets or chests when needing a specific legendary. The loot pool would be so diluted compared to the boss any given legendary is assigned to, that you could farm for months on end and never have it drop. If i wanted an infinity, I would only farm doc. Nothing else is worth it because it would just be too random. Infinities are garbage anyway and have never understood the fascination surrounding the weapon. Ammo is plentiful and I can only think of a single build (Zero) that it becomes semi useful.
Loot midgets are good for specific pearls and artefacts, nothing else.
Suck it up and use another gun. Also you can Rocket Jump to Doc Mercy in Digistruct peak for three tries to kill him, slightly faster than jumping in via the Geyser.