As you progress through the Shinra Building in Midgar, you will reach Floor 63. You can obtain up to 4 special items on this floor. Here I explain how to obtain all 4.
First, head to the control room at the lower-right part of the screen, and use the computer. You will be told you can open up to 3 doors.
Open the uppermost door to the left, then the 2nd door down. This allows you to access the left-side item room and pick up the Coupon there (the entrance to this room is at the top). Then crawl in the air duct, come out in the room to your right, and pick up the next Coupon. Leave this room and open the door to your left to access the middle item room, and pick up the Coupon here. Then go back to the room on the right where you've already picked up a Coupon, and take the air duct to the control room. Exchange your Coupons at the machine - you will receive a Star Pendant accessory, 4 Slots armor, and an All materia!
Remember, if you make a mistake, you can return to the Control Room and reset by selecting "All Clear" at the computer. You can reset the room as many times as necessary. However, note that you can't reset after performing a Coupon Exchange.
One last thing: if you return to the Shinra Building in the Return to Midgar at the end of Part 2; you can find the 4th item, the Grow Lance (a decent weapon for Cid), in Floor 63.
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