Driving north from the Interamericana through Zarcero, take a right immediately after the church and continue northeast about 15km. Alternatively, take the road due north from Sarchí's central plaza. Both roads are almost entirely paved but involve stomach-churning steep climbs and hairpin turns; a 4WD makes all the difference to your nerves but isn't obligatory.
There's a bus from Grecia at 3pm every day except Thursday. The fare is around US$2.