Hi Ruth Ann,
You didn't say how many are in your party, but I will suggest that with more than one person, yes, do book ahead in SJPP. The statistics for September for the past few years have shown a consistent increase in the number of people starting that month, especially the first week. In some ways, September is the new July.
As for where to stay, if you want a hostel (gite in France, albergue in Spain), L'Esprit du Chemin is a fantastic place to start your Camino. They allow pilgrims to stay for up to two nights, but you must book ahead, and you must conform to their arrival and meal times. Their website shows they are sold out for your dates, but you might be able to get on a waiting list.
http://www.espritduchemin.org/en
If that doesn't work, I can personally recommend Gite Azkorria:
http://www.booking.com/hotel/fr/gite-azkorria.en.html Booking.com shows them already sold out for your dates, but you can send them an email to request beds in the shared dorm:
gite.azkorria50@orange.fr
Another nice place in SJPP is B&B Errecaldia:
http://www.errecaldia.com/
As for Orisson, yes, do book in advance, if they have anything left. If they are fully booked for your date, you can ask for beds at their new gite, Kayola, which is just 800 meters before Orisson. And a final option is Gite Hunto, which is 5km after SJPP.
Good luck!