Specialty board game stores are your best bet. Look in local hobby shops and board game stores, because many of them stock replacement parts for classic games like Carcassonne and can order the tiles you need. You can also try online stores that specialize in board games in Spain or Europe, which usually have wider catalogs than Amazon for this kind of stuff. Some official distributors also sell replacement components directly.
Another option is to contact the game's publisher or distributor in Spain. They normally have customer service and can sell you individual pieces if you tell them exactly what tiles you need (terrain tiles, abbeys, meeples, whatever). It's also worth asking in online board game communities, because sometimes other players have duplicate pieces or know about specialty stores that don't show up on Google.
If none of that works, there are 3D printer shops that sell custom board game pieces, though that would be more for very specific tiles. But seriously, start with your local game stores - that's usually the fastest and most reliable option.