Right I've found an Accessory Specific Guide that, hopefully, details everywhere you can get a Ribbon from.
I'm copying this bit of the guide relating to a Ribbon directly so the credit goes to a member of another website called Sephirusoy

I really hope this helps.

Note: The maps have been linked directly for you to look at as we don't give links to other gaming related sites.

Jay
Site Squad
p.s. Your best bet is probably trying for the Lv99 Red Chocobo as it's easy to get a chain of the Black Chocobos just outside the Garif Village of Jahara (in the Ozmone Plains).

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Ribbon = Immune to many bad statuses
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1. Treasure in Cerobi Steppe/ Crossfield
   • Story reached Balfonheim.
   • Equip Diamond Armlet.
   
   
   
2. Treasure in Subterra
   • Story passed Pharos at Ridorana.
   • Take the Dais of Ascendance at 1F to Subterra.
   • Fill enough Black Orbs to each altars in B1F (Penumbra) to unlock B2F.
   • Fill enough Black Orbs to each altars in B2F (Umbra) to unlock B3F.
   • The treasure is in the North-West room of B3F (Abyssal).
   • Do not equip Diamond Armlet
   
   
   
3. Treasure in Henne Mines
   • Clear Mindflayer rank B/IV mob hunt.
   • Collect 10 Espers or more.
   • Talk to the geomancer in Jahara to unlock the secret area.
   • Find the treasure 2 display in the map.
   • Do not equip Diamond Armlet.
4. Drops:
   1. Lv99 Red Chocobo ............. Ozmone Plains/ Haulo Greens
      • Chain the chocobos in the sections near Jahara.
      • Lv99 Red Chocobo has a low chance to show up after defeating all chocobos around.
   2. Vishno ....................... Pharos at Ridorana/ First Ascent
      • It's a rare monster, will have a random chance to show up after defeating certain amount of Brainpan and Deidar.
      • And the chance of dropping Ribbon is very rare.