Hi Judy,
Paris to Bordeaux by TGV train is a very popular route, thus, you'll have no problems finding a ticket.
Normally reservations for trains cannot be made more than 60-90 days ahead of time, so you'll have to wait a bit before making reservations.
Where to buy tickets online depends on where you're coming from. If you're coming from USA, you'll need to go to
RailEurope.com to buy the ticket. England:
RailEurope.co.uk. Canada:
RailEurope.ca. Elsewhere:
TGV-Europe.com.
I just checked on RailEurope.com and prices for October trains between Paris
Gare Montparnasse and Bordeaux are $38 usd, one way. The unfortunate part is that ticket delivery to overseas (Canada/US) is expensive and the only option. For the USA it's $18. Canada: $23. UK: free for ticket pickup at the station.
Best regards,
Ben