Rail - Toulouse to Seville
Checking with voyages-sncf.com shows that tickets for the TGV/AVE routes for France/Spain can only be booked three months in advance.
A sample rail itinerary generated today for Thursday, 30th March, 2014 show the following:
Depart: 05h50 GARE DE TOULOUSE MATABIAU
Arrivée:07h15 BEZIERS GARE
Départ: 09h18 BEZIERS
Arrivée:14h13 GARE MADRID ATOCHA
Départ: 15h00 GARE MADRID ATOCHA
Arrivée:17h20 GARE SEVILLA SANTA JUSTA
Price: 191.50 Euros.
Air:
AirFrance.com operate direct flights between Toulouse and Sevilla several times a week, Booked well in advance prices are not unreasonable. 
Vueling.com also operate via Barcelona but it may mean having to spend a night in Barcelona.