The best, most up-to-date information is going to be in academic books and articles, not a free online website. But you should be able to access this information without having to shell out cash.

The best book on the subject, which is both well-researched and readable, is Guy Halsall's Barbarian Migrations (2007). Your local library might have it (free), but if they don't they should be able to ILL it for you (which is usually free). That's where I'd start.