Having completed Xenosaga II and thus enabling the Vector Uniform, I am finally about to undergo my perfect Xenosaga III save. Although I hadn't played XenoII until recently, I had played Xeno I and III a ton so I'm pretty familiar with them. However, to the best of my knowledge there isn't a complete missable item list anywhere, just some general guidelines. So I'm hoping to compile one both for my own benefit and that of others, in chronological order.
* Vector Uniform (must import with Xeno II data) * Honey Teddy (quest on Fifth Jerusalem) * Dive Teddy (quest on Fifth Jerusalem) * Green Oasis (steal from T-elos in Floating Landmass) * Carnelian Egg (technically not missable, but don't sell them) * Kajic Neck (steal from Mai in Dabrye Mine, Old Miltia) * Prescription Copy and the shop it unlocks later in the game (sidequest, Old Miltia. Must complete before fighting Voyager.) * Venom Ring (steal from Voyager on Elsa, Old Miltia) * Lemon Ice Cream, Old Miltia * Berry Ice Cream, Old Miltia * Melon Ice Cream, Old Miltia * DX Ice Cream, Old Miltia side quest - Must knock down the hood on the ice cream vendor's cart and then open it again once the assault begins. * Double Vestment (steal from Pellegri in Labyrinthos, Old Miltia) * Union Neck (steal from Virgil) * Crescent Moon (steal from Citrine on the Durandal) * General's Bracelet (steal from Dmitri Yuriev in Abel's Ark) * Life Demon (steal from T-elos in Underground Ruins) * White Shirt (steal from Kevin in Underground Ruins) * Research Uniform (steal from Shion in Underground Ruins) * Velvet Pannier (steal from Kevin in Underground Ruins) * Medicine sidequest (Old Miltia)
Do let me know if there's anything missing. This doesn't, of course, account for the items in shops that no longer appear later in the game. It would be awesome if I could compile a "deadline list" for items in shops so you know when you need to get them by.
It lists off what items are available for purchase in chapter 9 from the endgame shop plates, assuming you have done the Seven Moons sidequest (which I feel is presumably a safe assumption).
Anything not listed in either the first long list, or boldfaced in the shorter second list, you should stock up on earlier. You could make an exception for things buyable from that one computer terminal available by EVSing to Miltia (notably Seven Moons), but you might as well just get them at the time and not have to worry about whether or not they'll be available later.
Incidentally, that sidequest is available longer than you describe. I think it's available at least through chapter 6, and also through the beginning of chapter 7 before you get Shion back. (In other words, it's available so long as the town is relatively peaceful.) --- - Vilurum Xaren, leader of the happy turtles, and knowitall. ... And if you can't spell "Vilurum", call me "Vil". :)
I have a basically complete missable item list (including shop items) compiled, which I'm checking against the game as I do my perfect save now.
I've run into my first discrepancy in Chapter 8. The Item FAQ on GameFAQs lists a G-Rox/70 E.S. generator that becomes available after the Durandal events and is no longer available in Chapter 9. However, I am looking at the blue plate now, both in the E.S. and inside Abel's Ark, and I don't see it available. i also can't find any reference anywhere else to a G-Rox/70. Is this a mistake, or did I already miss my opportunity?