I can highly recommend Hope for Dogs in Murcia. Jodie and her team are absolutely amazing. They work tirelessly to rescue puppies and dogs of all ages in some dreadful conditions.
Also if there is a specific dog that you want she will try and get it for you.
Have a look at her Facebook page.
There's also Costa Blanca Dog Homing, Akira, K9, Paradise Dog Kennels, Pets in Spain the list is never ending these are all on Facebook and are all charity based
If you want to foster then they will need to do a home check which is straight forward and they will even deliver the dog for you.
I went to Torrevieja Dog Pound as advised on here we chose a dog asked the girl if she was ok with other dogs she didn't know "we can try" she said so waited outside with my younger dog till she bought her out.
All i can say is thankfully the other dog was wearing a muzzle and harness I dread to think what would happened if it hadn't.
Be aware there are alot of pitbull/pitbull mixes in there so unless your willing to go through the process of owning a PPP dog (which i have done) I wouldn't bother it's not worth the hassle.
I love my PPP dogs and don't know why some dogs are labelled this mine was soft as putty.
There are plenty of dog rescue centres that you can try.
Also if you do decide adopt/foster (if there is such a thing as foster I am a failed foster twice lol
)if you take the paperwork to Benican Vet in Benimar you get 10% off all products and 20/30% of all vaccs and operations
Good Luck in your search and I hope you can give one of the many dogs out there a forever home ❤