I'm a Legend: Achievement STEAM of Assassin's Creed Origins

A fast way to get legendary (yellow) gear is from Papyrus Puzzles. There are 25 of them in the open world and each one can give a piece of rare (purple) or legendary (yelllow) gear.
See Assassin's Creed Origins Papyrus Puzzle Solutions & Locations.
You also get legendary items from loot chests and by defeating enemy captains. Odds are that you will get enough legendary gear automatically during a normal playthrough. If you want it early, go solve a few Papyrus Puzzles for the gear. Killing Phylakes also has a high chance for legendary drops. Always keep legendary gear and don't sell it.