SNCF is notoriously difficult to get online bookings but I believe that as long as there is an available seat they will sell you a reservation on the day. Also there are several TGV routings that could be used.
Renfe are even worse and you can only get reservations in person in Spanish stations, but there are plenty of trains BCN to MAD.
The direct services from Paris to Barcelona (a joint service with SNCF and RENFE are expensive and very busy and you may want to consider alternative routings.
Use a number of planners to develop your routings.
Consider routings using regional trains - longer times but no reservations needed.
Best combo of time and lower cost is the NIGHT train P→ Port Bou (or Cerbere)-these are the stops just IN or before the border from FR to ES. Forget that fast TGV-superexpensive and among the most hard to get pass-seats on. The local rodalies/Catalun expreso to Bcn and AVE to MAD-needs 2 traveldays that way. Nighttrain also has couchette=basic bunk, cost 21,50. IF you happen to be in FR a few days before-best way is to try at a SNCF machine-it needs quite a few steps, but will work reliably.
Surcharge for direct TGV P-Bcn is around 40€
Surcharge for direct TGV P-Bcn is around 40€
@mcadv it's €34. If you're not sure, then please look it up.
Delighted to say B-europe recently supplied me with e-reservations for TGVs with no problem. One journey gave the 10 euro rate and one the 20 euro rate, Nice - Paris and Paris - Lille.
Also supplied Eurostar from Lille at relatively short notice (Paris was not available - hence the TGV from Par - Lille.