Even a subscription to the online version of the Dictionary of National Biography is beyond the means of most of us, let alone buying the book version. Those with access to a good public library service may find that it has an institutional subscription.
However, the DNB does put up daily specimen articles, which you can view for free for a week at a time, and today's article, which will be up until Saturday, is on Magnus Maximus. See also the DIR article, possibly written by a Mel Gibson fan.
Magnus Maximus features in Gibbon's chapter XXVII, which we will be discussing on 11 July 2007.