how do you get the Ultima weapon or Ragnarok on final fantasy 12?

The Ragnarok is given for defeating the mark Ixion. This is towards the end of the game and the mark is found in the lower areas of the Pharos (so you need to be at a good level, ideally around 65 at least).

Ultima Blade is also in the Pharos subterra. You can find it in a chest in the north east corner of the second level (umbra). There is a 30% chance of the chest being there, and if it is it will have the sword in it. If it's not there then go two screens away and return until it appears.

The Ultima Blade can also be created in bazaar goods, but it might take you a while. To do it you need the following:

2 Adamantite

2 Death Powder

1 Gnoma Halcyon

To get them:

Adamantite - Adamantitan on Cerobi Steppe. 8% chance of dropping one (must have hunter monograph). Also steal from Thalassinon rare game (Phon Coast, Vaddu Strand - must wait 10 seconds at far east section for it to appear). Also defeat mark Darksteel.

Death Powder - Dropped by Bogey in Zertinan Caverns (Drybeam Cavern, Darkened Wharf areas) with a 16% chance, must have warmage monograph. Also steal from Ahriman in Sochen Cave Palace (Hall of Shadowlight area). Also you can poach Cultsworn Lich in Tomb of Raithwell (this would be tough so I would get it from the others)

Gnoma Halcyon - Rare drop/rare steal from Gnoma Entite in Dalmasca Westersand (Midfault, Shimmering Horizons areas). As you probably know, these things are tough so be prepared.

Ragnarok Final Fantasy