Find below some general links about accommodation in Bilbao, which can be directly booked by you:

Solidary accommodation

KUVU
Bizikideak

Residences:

http://www.atalaiaclaret.com/en/index.html
http://www.bidealde.com/I_principal/Index.htm
http://www.bilbaobbktalent.es/inicio/ (Exclusively for highly qualified researchers)
http://www.ikasleak.ehu.es/p202-shunamhm/es/
www.lasaileku.com
https://micampusresidencias.com/es/residencia-universitaria-bilbao/
http://www.nsdelrosario.com/
https://www.resa.es/en/residences/bilbao/residence-hall-blas-de-otero/residence/
http://www.residenciamariainmaculadabilbao.com/
http://www.residenciasantisimatrinidad.es/content/por-que-elegirnos
http://www.riabilbao.com/index-i.asp
http://www.fadura.ateri.net/ (for affiliated sportsmen)
https://www.resa.es/es/residencias/bilbao/residencia-universitaria-san-mames/residencia/

Youth Hostels, Hostels and Guesthouses:

http://albergue.bilbao.net/
http://www.allironhostel.com/
http://www.bbkbilbaogoodhostel.com/?lang=en
http://www.bilbaoakelarrehostel.com/cas/site/albergue.asp
http://www.bcoolhostels.com/hostels/bcool_bilbao/
http://bilbaocentralhostel.com/index.php/en/
http://botxogallery.com/
http://www.ganbarahostel.com/pub/en/
http://www.la-estrella-ostatu.com/
http://www.moonhostelbio.com/
www.pilpilhostel.com
https://www.poshtelbilbao.com/?lang=en
http://www.telleri.com/
https://www.quartierbilbao.com/en/home.html

Flats and Apartaments:

http://www.alkila.net/
www.alquilerjoven.com
www.apartamentosatxuri.com
https://www.airbnb.es/s/Bilbao
http://www.bilbaoflat.com/
https://badi.com/
http://www.easypiso.com/
http://www.edificiosantiago.com/
https://erasmusu.com/es/erasmus-bilbao/alquiler estudiantes?utm_source=estudiantes.deusto.es&utm_medium=ac-page&utm_campaign=deusto-ac-oct19&coupon
http://habitaccion.com/
https://www.holidu.es/s/espana/bilbao?o=8717299
https://www.hometogo.es/
https://mendialai.com
http://www.pisocompartido.com/en/renting-homes-bilbao
http://spain.gabinohome.com/
https://www.uniplaces.com/accommodation/bilbao
http://www.wimdu.es/
https://www.swiftflats.com/es/ 
https://erasmusu.com/es/alquiler-estudiantes-erasmus