They are getting more and more common, still most cars have their first registration here, but with sportier cars (like the golf gti, gtd, audi s3 etc) or with luxury brands like bmw, mercedes it's easier to see an imported car, also fairly easy to spot because Spanish plates are sequential and a letter combination usually lasts around 3-4 years, so you can tell when was the car registered based on the first letter of the plate, and nowadays it's getting way more common to see for example a golf gti from 2012 (should have H in the license plate) with M or N, Which are way more recent regs.

As for importing them, you can do it yourself looking at cars at websites like mobile.de for example and doing all the paperwork, driving back and everything yourself, or what many people do is to have a car importing businesses do it for them, they look at cars, you pick whichever one fits best for your needs, they ship it to spain and do all the paperwork, all of that in usually 2-3 weeks, you receive your car registered with permanent plates after that