The International Leonberger Union does not currently keep a database. There are several dependently maintained databases on the web.

Worldwide Independent Leonberger Database
leonberger-database.com/

Leonberger Club Databases
Czech Republic
Great Brittain

National Kennel Club Databases
Austria: https://oekv.at/de/referate/zuchtbuchreferat/
Finland: https://jalostus.kennelliitto.fi/frmEtusivu.aspx
Italy: http://www.enci.it/libro-genealogico/libro-genealogico-on-line
Netherlands: https://www.dutchdogdata.nl/
Sweden: https://hundar.skk.se/hunddata/