Hotel search engine with also a wide range of apartments offered. This site searches through all majord accommodation sites and always returns the best price. Hosts can add their properties for free, guests gets a price guarantee on bookings.
After making your search, use the filter option and choose "apartment". Hotelscombined is a great tool to compare Airbnb accommodation with other alternatives (hotels, motels, other apartment providers etc) in the area you'd like to stay.