Resplendent sparks are currently found by either salvaging a Mythic Unique or by making a new character and farm up the Reputation with the Iron Wolfs in Hell tides.
I think there should be more ways, and this is just a suggestion: have a grindable currency even after you are max rep with the Iron wolfs and level 100 with your character. So you can keep playing your main character if you prefer to do that.
For example: Each time you earn the amount of Xp corresponding to going from lvl 99 to 100 you earn some currency reward. This currency reward could be turned into a resplendent spark at the Alchemist after collecting enough of them, perhaps 50 or 100 of them, or any other resonable time investment.
It should take long time though, So it is around the same time as getting a new character to max rep.
This would give everyone something to grind for even with a max level character, and also give everyone some kind of “Bad luck protection”. As it means even if you never get the uber you are looking for as a drop, it would eventually come this way.
The currency earn this way could also be used for other things, like you could turn them in for a cache with random items, cosmetic rewards or other things players want to gather.