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.
3rd Singularity Okeanos: Caldera Island free quest is the first location you can farm Meteor Horseshoe.
Here is a list of servants that need Meteoric Horseshoe to perform Ascension and Skill Up.
Rashoumon event (Ibaraki Douji event)
Murder at the Kogetsukan
Inheritance of Glory
5 Star Servants Tier List: Best Servants [5/9 updated!]
Best Craft Essence: Craft Essence Tier List
Minamoto No Yorimitsu (Lancer, Summer 2017)
Best Servants [★4]: 4 Star Tier List
Free Servant Guide
5th Singularity: E Pluribus Unum
Marie Antoinette (Rider)
Spring Training of Tenma
Talk on the Heated Sands
Elixir of Love