You need to farm legendary items. To get a chance to get a legendary item you need at least 50k+ points in a ride or daily ride, if you get 250k+ points its a guaranteed legendary item, free roam and bike parks don't count. Easiest way to get points is to spam tricks in the desert while on last chance, if you suck at doings trick you could just get the perk to do fakie and spam flat maps. Its really time consuming and not really worth it unless you really want a certain items or you want to do the achievement to get the whoopie cushion.
