I had meant to produce a piece with details of the main bus routes and timetables but these are difficult to find opine at the moment. Perhaps, the summer timetables have not been released yet
However, I’ll go over the basics again.
You book your tickets at train stations or bus stations. Often if you are in train station you can buys ticket that combines rail and bus travel. For example, travel from Pau to Cauterets and your ticket will cover you from Cauterets to Lourdes (train) and then Lourdes to Cauterets (bus).
In the High Pyrenees buses run direct or change at the village of Pierrefrites.
Valée d’Asp — train to Oloron and then bus.
Cauterets — bus from Lourdes (outside of train station)
Gavarnie — bus via. Luz St Saveur. The small mini bus that runs from Luz to Gavarnie is not on the SNCF timetables and I don’t thick you can buy a through ticket.
Buses from St Saveur nearly always travel on to Bareges.
Bagneres du Luchon is also served from Lourdes.
I’ll post more details when they become available.