If I remember correctly, Garuda can be obtained at Mt. Matoya's summit - Mt. Matoya is located on the northeastern (Lenadia) continent (north of the Wayfarer's Rest). Compiling a comprehensive list would be a lot of work, though, so I doubt one will be available soon (or at all). Exploring all locations on your own is half of the fun, so just search through every available location, maybe with the help of the Overworld Maps that can be found in the game's "maps and guides" folder.