the w-item materia was at the opposite end of the train tunnel in midgar when you went to stop hojo (the way everyone told you not to go). if you didnt get it then, there is no way to get it now ever. (besides cheating...)

the trick: use w-item. use one item, then select a target for the second item. then cancel. you will have one more of the first item. repeat until you have as many of the first item as you want

as for the limit breaks, im no expert on that. perhaps someone else can tell you.

someone made a thread asking for help in the battle arena, but im not sure where exactly it is now in the ffvii section...but it would help you in getting omnislash.

(and yes, with a hero drink, even at your level, omnislash could very well take out Sephiroth in one use. ultima weapon would help alot though.)

oh yeah, and dont worry about supernova. it cant actually kill you. though it will put alot of status effects and take down some 95% of your current hp...naturally, recovery would be in order.