I'm looking for a car rental company in San Diego airport that would have a specific model which is usually rare for rentals to have (Mazda CX-5). Anyone knows if any company holds them? Otherwise what is the biggest rental company in the airport so I can maximize my selection?
Almost all major rental chains will NOT guarantee a specific model. One exception is Hertz, who offer this as a special service and typically at a very steep price. link
You may not even get a car in the same category that you booked. It's fairly common that they are out and need to give you something different, which is often combined with an upsell attempt. Never accept a downgrade, never pay for an upgrade.
You may have better luck with a "broker" company like turo.com. They basically offer privately owned cars that people rent out on occasion. They have plenty of availability at San Diego Airport. Make sure you read the rules carefully and make sure you are ok with it. Many owners put trackers in their car to protect them from abuse. Pick up process can also be different from a regular rental, so, again, read the listing carefully or just ask.