Hi
@WayneShepard
You can maximise your options by searching for trains from Paris Montparnasse to Bayonne. In Bayonne you switch to a local comuter train for which advance bookings are not required and an attempt to secure one will probably b*gger up your booking attempt
Having travelled from Paris to SJPP many times, I agree with
@Tincatinker Focus on booking your travel from Paris Montparnasse to Bayonne. There are sometimes a few direct trains, but most stop in Bordeaux - some with a change of trains, others not.
If you include SJPP in your ticket (either from Paris or from Bordeaux), you may find that closer to the date your trip is 'cancelled', whereas in fact the only leg 'cancelled' is the very short trip (1 hour) from Bayonne to SJPP. And, in that case, there is almost always a replacement bus provided. That is not uncommon on the line between Bayonne and SJPP and it pops up regularly on the forum.
You can buy a separate ticket from Bayonne to SJPP - usually less than 10 euros, or buy that on the day.
I would book with either Trainline or SNCF. Sometimes when booking from outside Europe, Trainline can be an easier option.
Also, from Bayonne, you have the option of travelling to SJPP with Express Bourricot, though you would arrange that ahead of time.
All the best.