New Bus Station at Jalan Pasar.
Same row with the Train Station.
FYI, the road from SP Police Station to Pekan Lama already closed, so you need to take the next route, from SP clock tower heading to Taman Ria, take the first traffic light to the right after the bridge, then go straight till you find the KTM, Pasar Ikan and the Bus Station on the left side of Jalan Pasar.
Sungai Petani has no airport. It has one bus terminal cater for short journey trip buses. The bus terminal is in the centre of the town. If you want to travel to nearby towns or within the town well this is the place.
But if you are looking for buses offering further destinations, you have to go to "Jalan Market" (Market Road). You can find ticket counters selling tickets from different bus companies. To travel by bus in Malaysia is very cheap.
Local buses from Butterworth to SP and to Alor Setar..and to other small towns in Kedah.
The Bus station, for all I remembered, look the same from the 70's..