Pasta

Item Class  Name              Where to find?                     Thief level
=============================================================================
Fist        Ultimus           IG1 of a legendary God's Hand          1871 (1)
                              IG2 of another Ultimus                 3604
Sword       Yoshitsuna        IG1 of a legendary Cosmic Blade        1871
                              IG2 of another Yoshitsuna              3604
Spear       Glorius           IG1 of a legendary Longinus            1871 (2)
                              IG2 of another Glorius                 3604
Bow         Galaxy            IG1 of a legendary Artemis             1871
                              IG2 of another Galaxy                  3604
Gun         Etoile            IG1 of a legendary Infernal Gun        1871
                              IG2 of another Etoile                  3604
Axe         Apocalypse        IG1 of a legendary Durandal            1871
                              IG2 of another Apocalypse              3604
Staff       Omniscient Staff  IG1 of a legendary Galactic Staff      1871
                              IG2 of another Omniscient Staff        3604
Monster     Nyanko Soul       When Majorly joins
                              IG2 of another Nyanko Soul             3604
Monster     Nemesis MK I      Steal from Baal                        2139
                              IG2 of another Nemesis MK I            3605 (3)
Monster     Nemesis MK II     Steal from Prinny Baal                 3140 (4)
Armor       Astro Suit        Steal from General Carter               247
                              IG2 of another Astro Suit              3604
Armor       Super Robo Suit   Steal from Baal                        2139
                              IG2 of another Super Robo Suit         3604
Armor       Prinny Costume    Steal from Prinny Baal                 3140 (5)
Engine      Hyperdrive        Get it by beating your first IG2       3604 (6)
                              IG2 of another Hyperdrive              3604
Accessory   Gao's Guts        When Majorly joins
                              IG2 of another Gao's Guts              3604
Accessory   Myao's Cat Ears   When Majorly joins
                              IG2 of another Myao's Cat Ears         3604
Accessory   Crowdia's Beauty  When Majorly joins
                              IG2 of another Crowdia's Beauty        3604
Treasure    Arcadia           When Majorly joins
                              IG2 of another Arcadia                 3604 (7)

Notes:
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(1)   You can steal a God's Hand from the Zombie King in Cave of Ordeals 5.
      Note that its rarity is random, so you may want to save and reload
      until you get a legendary one (1/32 chances)
(2)   You will get a legendary Longinus for reaching level 25 spear mastery
      with one character, entering the Dark Assembly, then speaking with the
      Longinus character in your castle
(3)   To be confirmed - I don't know for sure whether the IG2 here is also at
      level 6933, or higher.
(4)   Nemesis MKII is already at level 100 when you get it
(5)   Prinny Costume is already at level 100 when you get it
(6)   It will pop up in your inventory without notice, so make sure you have
      some space left.
(7)   In theory, you could get an Arcadia from the IG in an exodus, but it's
      supposedly extremely unlikely. I've never seen one myself.