You can also buy the reservation in international counter at many stations in Europe like in France at SNCF or in Germany at a DB Reisezentrum. 

You can also try to google some travel agency in other countries that sell SNCF, Renfe or DB Tickets, they are able to sell the reservation too (if they know how to book it).

Or you take the regional trains (no need for a reservation) from Barcelona to Cerbere or Port Bou and than change there for an other regional train to Perpignan there you can take the first TGV that you can book to Paris online at the interrail.eu reservation service. For TGV within France you get a e-reservation.

Thank you Angelo brilliant response better than Interrail.if I get stuck then I will go regional but as going to spain will try Stations like Seville / Madrid / Valencia or Barcelona.

I supposethat if I am taking an intercity train that needs a reservation in Spain I buy it at the train station.

Many thanks