I know there is definitely a bus from Plovdiv to Pamps but where from I am not to sure. If I was you I would phone a tourist information desk in Pamporovo. They should speak English and should be able to give you the details you need.
Alternatively if your flight co-insides with a travel companies try to hitch a lift from one of there transfer buses that will be going from the airport straight to Pamporovo. Hope this can be of some help.
I am getting the train from Sofia to Pamps. Have been told the buses leave on a regular basis and the train and bus station are side by side. My friend landed recently at Plovdiv (diverted from sofia due to fog) and had problems getting a taxi. When they eventually did he stopped 3 times on the way for coffees