How to find big groups?

How to find big groups for spamming (sharing posts from my fan page with my website)?

Has any seance to spam in close groups?

Why not create your own group or have you tried joining groups related to your niche? If you do join a group or create your own group "over spamming" or "bombarding" people with unrelated posts or content to try and drive more traffic will not be of any benefit to you. You'll either get kicked off the group or people will leave your group.