Cheaper ways:
For MALE Char
It depends on what your current character is in the game. for male characters it has the "Dress Plate" armor that gives you a very high resistance to "Dark". Her defense is really bad. however.. in addition to the high resistance to dark, it does not require any level to equip it.
All chars Lv80:
Anti Dark Ring, with this shield equippable at level 80 you will have +60 resistance against dark.
Gratia (Shield for casts Lv.101 req.): All resists = 34/34/34/24/24 (dark24)
Three Seals (Shield for FOrces lv.33 req.): All resists = 33 (dark 33)
Ylvis Custume (Red Ring [need lv150]+ some "Halo Rappy Souls") Resists = 30/20/30/25/0
Centurion/Resist a non very expensive unit that give u All resists +15
Adept "Adept" is not focused on resistance but adds shield, armor and other resistance units along with the perks of improving stat and ATA. It's a unit I highly recommend, even better if you can use techs. Because it reduces 25% of spending to use tech.
And theres some another equipments that u can use too.
Not counting something so obvious the best armor in the game STA that gives you an insane +80 resistance in everything. But very expensive.