6th Singularity Camelot: Land of Emptiness free quest is the best AP & time-saving location to farm Meteor Horseshoe. You'll get about 4~5x Meteor Horseshoe by spending 220AP.
Okeanos: Cladera Island
3rd Singularity Okeanos: Caldera Island free quest is the first location you can farm Meteor Horseshoe.