Where can one get form E101?

Form E101 “Certificate of applicable legislation” is issued by the competent institution of the state in which the worker retains welfare benefits (it certifies that the worker retains entitlement to the social security benefits of the country).
     
The contact details of the competent institutions or authorities with regard to secondment are available from CLEISS (see CLEISS’ contract details in section Useful links , check section “Documents” then “Social security organizations” on their site).