I mean, this isn't exactly
difficult as much as prohibitively time-consuming, and nobody is ever going to do it again because there's no point in it but...
Back when Abyssea was the activity of choice, when empyrean the hot new thing, you would have to go around and fight NMs for your seals, make sure you get the yellow proc, decide who gets what, all that. I actually established my current triobox setup because I wanted to be able to cover every yellow stagger - blm/brd, blu/nin, and whm/rdm if I remember correctly.
(Incidentally my blue mage, Merantha, did the 75 limit break for the first time on blu long before trusts were a thing, I've heard that was a bit of an accomplishment but it's hardly special.)
If you didn't get to do the right NMs, or other people were ahead of you in the lotting queue, or you just had bad luck with it, though, there was another option... the
quests.
Each seal had I think two quests associated with it. You'd have to do the hardest version of the quest or complete it without error to get a
chance to drop one, and then if you did, it was like a 10% drop for one of four random seals. Not a realistic option, really; it was just a scrap they tossed us for picking up an extra seal if you really needed it.
I
completed all five pieces of my caller's attire +1 set with seals gathered exclusively through quest completion. I contributed something like 200 data points to each quest's drop table on FFXIclopedia, but sadly that hasn't survived the turbulence that wiki's suffered over the years.