it's better look for KSRTC bus..
Kerala Road Transport (KSRTC) operates buses to Munnar from KSRTC Bus stand from 5:30 am to 3:00 pm, every 40 minutes.
Morning : 5:20 AM, 6 AM, 6:30 AM, 7:30 AM, 8:30 AM, 10 AM
Afternoon : 12 Noon, 12:40 PM, 1:40 PM,
Evening : 3 PM, 5 PM, 5:40 PM
KSRTC Bus stand at Ernakulam is a kilometer away from Ernakulam Junction reachable by pre paid autorickshaw.
Ordinary buses (not air conditioned) ply between Ernakulam and Munnar covering the distance in about 5 hours, most of it over hilly terrain.
If your train stops at Aluva, please get down at Aluva and board bus to Munnar from Aluva. Not reaching Ernakulam city will save you an hour and the bother of rickshaw ride from junction to bus stand. All buses from Ernakulam to Munnar go via Aluva thus frequency of bus at Aluva is same as that at Ernakulam.
You cannot book Ernakulam to Munnar bus in advance online, though you can buy token for the next bus once you reach KSRTC Ernakulam bus stand.