In Germany and the USA (and more), one can register with the dkms.de or www.dkms.org
In France, with the Agence de Biomédecine: https://www.dondemoelleosseuse.fr/se_preinscrire
In the UK, with the NHS: https://www.blood.co.uk/stem-cell-donor-registry/
etc.
All these organisations share registers and match donors and receivers across borders.