Apart from the JEUPSITE project (http://www.jeupiste.eu/partner-search). There are many tools available that you can use. First of all, it is helpful to join Horizon 2020 information days in Europe that are organised on specific themes/areas within the Horizon 2020 programme. These information days often have also a matchmaking event at which you can find partners.
Online tools are also available:
CORDIS Partner Service: https://cordis.europa.eu/partners
Enterprise Europe Network (EEN): open for all organisations in Japan and for research organisations and SMEs in Europe: http://www.een-japan.eu/. In the case of European SMEs seeking partners in Japan, please contact the local EEN agent that can be searched from the following website: http://een.ec.europa.eu/about/branches
Thematic NCP networks often have a separate partner search service:
You can also get in contact with the National Contact Point for Horizon 2020 in Japan (NCP Japan) to tap into these thematic NCP networks.
In addition, unofficial tools are also available such as various Horizon 2020 related linkedin groups and consultancy services.
For a full overview, please access: http://ec.europa.eu/research/participants/docs/h2020-funding-guide/grants/applying-for-funding/find-partners_en.htm