You can always seem to get tickets the day of the game, although I don't recommend a "scalper".
Even for rivalry games, like the one we attended between Valencia and RM a few years back, you can get "standing room only"-style seating. We actually ended up on the fringe of the Valencia section.
With Dépor coming into town, it should be a good match, but you might get lucky, and not have to pay inflated scalper prices, etc. by going to the taquilla for SRO seats.
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Ongi etorri!