Getting Dark Matters in Blitzball is the most boring way of getting them, i think. I played ffx for about 200 hours already and made for all characters the "best" armours, i mean with curseproof, deathproof, ribbon(i play PAL version) and break HP limit. All this abilities except deathproof(if i am wright), you can get battling Dark Youjimbo constantly. Meaning, kill him 4 times, then save then restart. 50 % chance that hr will drop armour with ribbon, and other three slots free. Again, it for diehard players only, cause after getting 99x Dark Matters after opening Ultima Buster, i killed "very many" monsters in monster arena to make 600x Dark Matter at all. Also you can kill Penance arms constantly, and then flee. Every arm if overkilled gives you 2x DM as common item. At the times of my second walkthrough i Zanmato'ed DShiva to get Destruction Sphere i missed earlier. As for DValefor, Bahamut couse of upgrade for Tidus. But at my third wlkth i killed all dark Aeons fairly. This was much more interesting, not saying about Penance, who took me about an hour of so. I killed DYojimbo for about 40 times to get good armours. By the way all my stats now are at 255 except luck(125 or so), cause of that damn earth eater who gives only 2 fortune spheres. (I mean its to boring to kill him). Avoiding thunders was really piece a cake for me, i just stood a little north of travel agency, where the two bolt zone.