Well, if you hope to find any G1 ponies... you better forget that idea, because there are no stores that sell them. Unless you find some toy shop that has been lost in time and that still has some old stock. I'm still looking for one of those myself.
But you might find G3s, G3.5, G4 and fakies. If you are very very lucky you might find G2 (I found some babies two years ago where I spend my vacations).
Flea markets are empty too. But give it a try, you might get lucky. And there is a toy fair every third saturday in Madrid, but I only found G1 on the opening day... after that, only G2, G3 and fakies.
Where in Spain are you going?