That's because they are the best. Use the W-Item trick as mentioned before. Buy 2 Sylkis Greens. Select one in battle and choose to use it on someone, then select the other Green. Cancel this, then re-select it. You should notice you have one more than you started with. Keep alternating between 'O' and 'X' until you have 99, then exit the item menu and kill whatever it was you were fighting against. This works with all items that can be used in battle.