So as it stands, the only way to obtain Diamond Arrows is through drops from some obscure monsters, and at a very very low rate. In GMS, you could purchase them from Mo after doing the Raven Ninja clan prequests. Could we have them implemented in some way, either to buy or craft?
As far as I know, the Raven Ninja clan shop will be implement sometime in the near future, along with the rest of the Raven Ninja content; please be patient. Diamond Arrows, another All-Cure Potion source and all of the Raven Weapons (including Balanced Furies) are much desired items and we are working on having them implemented as soon as we can.
Stop wanting to want Red or Diamond arrows. The millisecond you activate Soul Arrow, the effect nullifies. It only appears to raise your dmg in your Ability DMG range, but it shouldn't give the actual boost when you use Soul Arrow.
which doesn't really matter at all, because you can just... y'know... not use soul arrow, and then actually get the damage bonus.