Sooner or later you will notice in the field of legal history the Max-Planck-Institut für europäische Rechtsgeschichte at Frankfurt am Main. The Max-Planck-Gesellschaft is for most scientists and scholars a German institution known for institutes devoted to fields like molecular biology, but there exists MPI’s – the common abbreviation – for art history, the history of science, and for European legal history, too. The Frankfurt MPI is the home of a very well equipped library for its field. Its digital library offers many things, apart from the very detailed catalogue which yields more details than your average library catalogue. The Virtual Reading Room contains German books on civil law from the nineteenth century, a most important period for German law, and scores of law journals from this period. Many thousand old dissertations from the German Reich have been digitized. Of its own publications one can consult and download all issues of Ius Commune (1967-2001), and this is not the only journal published at Frankfurt: Rg-Rechtsgeschichte is the newest. Add to all riches (for example, the microfilms of manuscripts for both medieval and Byzantine law) of course a well organized and very useful link selection, and you will either visit often their web site or consider visiting Frankfurt.
In my post Revisiting Frankfurt am Main (September 2012) you will find a much more detailed and uptodate portrait of this institute.