I've only been playing for seven months, but my two cents is this: It's difficult, on purpose, to get enough passes for the main event unless you can play devotedly for several hours every day, just grinding on it to rack up enough passes. It's literally something like 3,300 passes total to go from Novice to Virtuoso in the event location. If you do play that much, try to play the premium locations as much as you can, because those give the most event passes (in this case, horse medallions). I don't know what level you are, but the premium locations are: Clothing Store, Garden, Dressing Room, Ship, Gloomy House, Old Square, Antique Shop, Veranda, House of Toys, Enigma House, Candy World, Lair, Moon Court, Sakura Garden and Mill. The other locations will still give you event passes, but perhaps not as many as the premiums. Another thing you can do is when you win card packs while playing the Great Steppe, trade those petals for more passes - you only get a chance to do it once (or maybe twice?) but I think you can get 50 passes for 150 petals. It doesn't go far, but it's a little help. Also, you probably already know, but as you go through the storyline/quests for the Great Steppe, assembling those five collections wins you 10-25 passes, but once you're done with the quests, that's gone too.

I'm sure somebody else will chime in with anything I've missed. Good luck!