Slash storm is dropped by monsters, and you can also find it in the Free market.
A semi-confirmed list of what drops it is
here.
Slash storm IS required to be maxed, because at level 70, you get a skill called Bloody storm, which is in combination with flying assaulter.
Flying assaulter -> Bloody storm -> usually Upper Stab.
You also need the Flying assaulter, and Mirror Image books when you hit level 70. The Flying assaulter book can usually be found for about 12m.
Also note, these non- Cash shop versions do NOT have a 100% success rate, rather a 50%.