Dutch opinion polls:
https://en.wikipedia.org/wiki/Dutch_general_election,_2017
French opinion polls: https://en.wikipedia.org/wiki/Opinion_polling_for_the_French_presidential_election,_2017
German opinion polls: https://en.wikipedia.org/wiki/Opinion_polling_for_the_German_federal_election,_2017
Italian opinion polls: https://en.wikipedia.org/wiki/Opinion_polling_for_the_next_Italian_general_election
EUROBAROMETER:
http://ec.europa.eu/COMMFrontOffice/publicopinion/index.cfm/Survey/index#p=1&instruments=STANDARD
http://ec.europa.eu/public_opinion/archives/eb/eb83/eb83_en.htm