It comes whenever the devs feel like it, which is not very often. That's why Maiming costs a stupid amount of platinum. Only a certain enemy drops the mod, and this enemy has to be hunted down as an event, sort of like an invasion, and then has a random chance to drop the mod you want. And yeah, the acolytes that drop Maiming Strike and Argon Scope don't even last a day from what I remember, so if you're not free when that happens, you're pretty screwed.
On the bright side, while Maiming Strike is ridiculously good, a
Condition Overload build is a worthwhile alternative for most weapons, unless they have pathetic status chance. It won't melt things with a slide attack like maiming does, but at least you won't feel compelled to have a slide attack macro to spam.
I know that doesn't solve your issue though. The acolytes are like Nitain, except taken to an extreme. If you don't have lots of free time or don't work from home, I have no idea how they'd expect you to farm this stuff.