I want to practice problems from national olympiads and to get a fell about how the problems are around the world I need your help. I found this site https://olympiads.win.tue.nl/ioi/noi/index.html, but some links are broken or unavailable. If you can shortly describe the process of getting to the national (like are there any regionals) and provide a link it would be very helpful.

I will update the blog with your comments.