If you're looking to practice disagreeing with people, then you could do worse than joining Hacker News, a community which very strongly rewards (well-cited, technical) disagreement.

See this post, where the top scored comment, (written by me) with 40 points, disagrees with the linked article, and has another dozen comments disagreeing with it.

Do beware being too negative too early, however. If your first comment is downvoted into the negatives, then your account will be hellbanned.