If you need to obtain all the email addresses of several websites you can do it manually one by one, but this is undoubtedly one of the most tedious and boring jobs that may exist.
What you really need is a tool like Tukanas Email Extractor, which in a fully automated way collects all the email addresses on the websites that you indicate, generating a text file with all the emails extracted.
Tukanas Email Extractor has several filters that prevent you from visiting the same website twice, even if it had a different domain.
You can free download Tukanas Email Extractor 32, 64 bit and safe install the latest trial or new full version for Windows 10 from the official site.