Rafael, Puna is right - don't try it - it's not legal. You really can get almost anything online. Also, quite a bit of the seed and plant stock in Spanish viveros and jardinerias come from other countries, so it's not all authentic Spanish.
I have several books on gardening in Spain (that's where I enthusiastically developed a green thumb!), and you may want to buy one while there. It could help you decide what you want and what it's officially called, so you could do a search for places here in the States to buy it from. One is called "Gardening in Spain" by Marcelle Pitt, published in Fuengirola (Malaga) by Mirador Books. It's in English. There are many available in Spanish.
If you do decide to visit a vivero - maybe to get ideas! - they tend to be located outside of cities, where people have yards. There are several big ones in the suburbs outside of Madrid.