My first thought was Ubeda. It's a very interesting city, but unfortunately not as easily accessible as many other spots in Andalucia. There isn't a local train station. I took a bus from Madrid; I assume one could take a train to Jaen and switch to a bus there, but a bus straight from Madrid (4 hours or so) might well be faster. There's also the Linares-Baeza train station, which is not in Baeza itself; there's bus service from there to Ubeda. Baeza is also interesting, but the ceramics are in Ubeda. I saw some very nice-looking, reasonably-priced ceramics in Ubeda. To give you an idea, I was tempted by some soup/cereal bows priced at about 8 euros each in 2019. There were some large plates that looked more one-of-a-kind (but may not have been, for all I know) priced above 100 euros. Overall, it looked like a very, very good place to shop if you liked that style of pottery. Not all the Ubeda ceramics are green, but it seemed that most are. Here's an example: https://www.rubylane.com/item/1573115-Ubeda-1/Tito-Ubeda-ceramic-plate?gclid=CjwKCAjw7p6aBhBiEiwA83fGuuZAjbCBZ6bv8Glg3tOySfak3qCEArJq7s0bpnNUiAN3xJeYWOgHNxoCdpcQAvD_BwE. That's a mid-20th-century piece, not current production, but it shows the color scheme.
It was my impression that sale of ceramics in Spain is rather regional. I don't remember noticing the green Ubeda ceramics elsewhere in Andalucia. I wasn't actively searching for them, but I do stop and look in the window of shops selling ceramics and glass as I encounter them. There's a ceramics museum in Seville's Triana neighborhood.
In 2016 I saw a shop in the very touristy town of Santillana del Mar (in Cantabria) that had some green ceramics; the shop may have been totally dedicated to ceramics. I wondered, after visiting Ubeda on my later trip, whether the Santillana products were from Ubeda. However, Santillana is really too far west of San Sebastian to be a practical target for you.
During a 2016 day trip from Barcelona to Cadaques, I stumbled on a couple of ceramics shops. One was down on or very near the waterfront; the other was along the walking route from the bus station down to the port. I don't remember what the ceramics looked like, but it didn't look like just tourist schlock.
I remember seeing at least one shop in Barcelona's Barri Gotic that carried ceramics; it may have been dedicated totally to ceramics. That would probably be the most expensive place to make a purchase, but ceramics are often moderately priced, and if you find something you really like, why not?
In case there are folks reading this thread whose itineraries include Valencia: The town of Manises outside Valencia has a long pottery tradition. There's a ceramics museum there. Honestly, I don't remember whether I saw ceramics shops or not, but one would expect to.
I think there's a very good chance a shop dedicated to ceramics would ship. What I can't guarantee is that your purchases will be adequately packed to avoid breakage. You should know that low-fired ceramics (and I think most inexpensive items are low-fired) tend to be more fragile than high-fired items. There's also the issue of shipping cost, which could easily exceed the purchase price of inexpensive items. If you're in the market for a 400-euro showpiece plate or something like that, the shipping cost would probably be more tolerable. It's also worth researching whether Spain (and any shop where you make a big purchase) allows VAT refunds on goods shipped back the US. For expensive items, the VAT refund might defray the shipping cost.