Jenna who is moving from the USA to Barcelona soon had some common concerns about gardening supplies in the old world. I will email you directly, but thought I would post this information about our experience and try to help others.
Seed sourcing for our Mediterranean Garden was a concern for us in the early days of our project. We knew that Spanish people had a passion for vegetable gardening and liked to see their land under cultivation. Balconies too were often adorned with beautiful flowers and foliage. Cut flowers and bedding plants were often plentiful. The few gardens it was possible to see into behind high walls or fences seemed to be well tended with specimen trees and shrubs. In a land less known, the availability of seeds from familiar and desired plants was an unknown quantity.
Our fear of not being able to find suitable seeds led us to begin by collecting seeds to bring with us; those seeds were purchased, or saved from our own garden in England. I even took to knocking on the doors of strangers and telling them of our proposed move, before asking me to take a sample of seeds from their gardens under their supervision. I did not get a single refusal, most of the people I spoke to were amused and generally delighted that they were able to help: ‘A corner of a foreign field that will be forever England’, perhaps. Once the garden project was underway, we started you may have read, with the blankest of blank canvases, I ordered some varieties from Thompson and Morgan who shipped to Spain.
Locally I found that DIY stores tended to stock a limited range of flowers and a choice of popular vegetable seeds. Like everywhere else if there is a demand there will be a supplier it is just a case of finding that supplier. In the centre of Figueres there is a small family run shop selling an extensive range of seeds and sundries, their staff are quite friendly and they are eager to help the customer. There are other agricultural retailers who supply plug plants for vegetable growers. In a hot country a greenhouse would be impractical and unnecessary for most of the year, so possibly raising tender plants is best left to professional growers. Though if you enjoy raising your own crops and flowers as we do you will always find a way grow your own plants even if it means finding shade for the less robust vegetation.
I remember meeting a Welsh man in Ibiza some years back, he told me that the best garden advice he had ever been given was that, if you are enjoying eating a particular tomato then save a little seed from the fruit by drying in the sun, and use that seed to raise your own plants. Likewise many crops provide seeds quite readily, beans are a good example; even those expensive organic ones.