BrickLink – http://www.bricklink.com
A secondary market for LEGO parts and sets.
BrickSet – https://brickset.com/
A “primary reference on LEGO sets on the internet,” per their About Us page. They conduct interviews, push product announcements, track conventions and news, and publish reviews.
Bricks & Pieces – https://www.lego.com/en-us/service/replacementparts
Shop for bulk parts or missing pieces from your newly acquired sets.