Fight Dark Yojimbo. You can fight him 4 times, then save and leave the area. When you come back, he'll be back to normal, meaning you can fight him 4 more times again. Don't fight him 5 times because that will permanently kill him. He drops 2 dark matter per battle, with a weapon or armour. While fighting him I got BHPL+Ribbon armours for wakka, yuna, auron, tidus, kimahri, and rikku and ribbon+3 empty slot armours for auron, lulu, yuna and Tidus. During this time I had collected enough Dark matters to customise armours for the other characters . It still takes a long time (about 10 mins per 4 battles) and it will take you 50 battles to collect 100 dark matters, therefore, 2 hours for each CUSTOMISED ribbon armour. But after each battle Yojimbo may drop the armour you wanted anyway, cutting the overall time.