In Chapter 3 - If you've sided with New Yevon, you have permanent access to Bevelle's dungeon thru whole Chapter. Just follow steps given above and get this Dress Shpere
If you've sided with Youth League, the access to Bevelle's dungeon is available only AFTER you clear Besaid and Kilika hotspots, but BEFORE you waste the boss inside Djose temple. Just follow steps given above and get this Dress Sphere.
Chapter 5 - Regardless of who you sided with, you have permanent access to Bevelle's dungeon. Just follow steps given above and get this Dress Sphere.
Chapter 5 - If you were unsuccessful in CH3, here's your second chance. Visit 'Luca - Basement A' and you'll find Shinra here. Challenge him to Sphere Break and win, but beware, he's tougher this time around - 20 turns, 30 secs, 100 quota.
Chapter 2 - If you missed it in CH1, go to Moonflow and attempt 'YRP, the scalpers three' subquest there. SAVE your game before doing this quest. You must sell at least SIX TICKETS WITH A PROFIT (breaking even will do it as well)! Once you make 10th pitch, you're 'teleported' to Tobli automatically. If you met the above quota, he hands over this Dress Sphere.
Now you need to go through the woods and find him, keep following him, eventually you will end up near a save sphere, to save alot of time and effert, i shall tell you where he is.
When you reach the save sphere near the enterance of the woods, head rigth off the screen, and right again on the next. On this screen, head up. and head to the bakc of the cave, there you will find O'aka. You now have 2 choices, either let the Al-bhed on him, or take him to your airship, it is vital that you take him with you, so do so.
O'aka will be aboard your airship from then onwards, he has a huge debt to repay. this can take along LONG time to do, but it is worth it beleive me.
Now there is where the cheating begins.. Once you have cleared his debt, he will sell his goods to you for a MASSIVE discount price, sucker.
Buy as many things as you can off him, then head over to Barkeep (yeah the bar to your left from O'aka) and sell all the items you brought off O'aka to Barkeep, you will make a profit. Repeat over and over until you are as rich you wish to be :)
Note, this cheat will no longer work AFTER chapter 3, as O'aka jumps ship and settles down in the tarvel agency where he first ran away from. So make the most of it while you can.
Enjoy :)