Here you go :

If you book when you arrive in Spain availability shouldn’t be a problem, esp. on routes with frequent trains such as Madrid - Barcelona.

Here's a list of known ways to book domestic trains in Spain:

You can also book all your Spanish reservations at the first Spanish ticket office you get to, e.g. in Portbou.

Also possible: go to a Reisezentrum in Germany. Yesterday I went to the one in Rheine , and within 3 minutes I had my two reservation for Alvia trains. Preferente (1st) class for €6,50 each.