Dododex
ARK: Survival Evolved & Ascended Companion
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Since nobody talked about it
The underwater caves on the "TheIsland" map are THE best way to obtain quality rex saddles, if you don't have that option I have a few recommendations:
Valguero: There a easy cave with no dinos where you can get 2 Yellow Loot Drops, 2 Blue Loot Drops and 2 Red Loot Drops, just make sure you have a flyer, a perfect one would be the Pteranodon
Ragnarok: Do runs in the ice cave, but make sure to grab the Loot Drops in the area before you grt into the actual caves, just lure the bears down the cliff if they agro on you
Abb: Well, basically just the Loot Drops sadly.
Extinction: Yellow, Red or Purple OSD, I recommend the Red one though
Gen/Gen2: Honestly, Pay2Win, just do the missions
Scorched Earth: Desert drops
Crystal Isles: Land drops located in the snow biome
Hope that this helped a few people, and it will take some time to get the saddles, just do not give up, it'll be worth it.