Hi MicMac
I don't know about mainland Spain or the other islands, but I often visit Menorca and there's an English lady out there who rescues and neuters cats on the island. Sometimes she has a stall in Mahon outside the American Bar selling items to raise funds or else it is possible to go out to her house near Mahon/Sant Luis to see the what she does and meet the kitties etc all with a view to raise funds. Like all other animal charities she's always looking out for any help and she can be contacted on
[email protected] if you were interested in helping. I hope you find this useful