Alternatives to AirBNB/VRBO for vacation rentals in Madrid

Try Booking.com

When researching a Spain trip in2017 I looked at alternatives. Googling short let apartments would bring up companies that offered them. My reservations, however, were lack of reviews and the cash requirements involved. Many wanted full payment prior to arrival and cash 'damage deposits' of several hundred Euro. I would then get it back in cash....but I had no need for that much Euro, nor was I happy with exchange rates X2 to buy Euro and then sell it again.
That was just my mind set, but there should still be companies around. You could also do some investigative wandering around AirBNB to see which hosts are actually companies and try to find them directly.