One possible way would be to contact the larger Esperanto libraries. Wikipedia lists them here.

The biggest collections of Esperanto literature are:

• The National Library of Austria (with the International Esperanto Museum).
• The Hector Hodler Library in Rotterdam, maintained by UEA.
• The Montagu Butler Library, maintained by the Esperanto Association of Britain.

The collection of The National Library of Austria has an online search which can be accessed here.

One possible way would be to contact the larger Esperanto libraries. Wikipedia lists them here.

The biggest collections of Esperanto literature are:

• The National Library of Austria (with the International Esperanto Museum).
• The Hector Hodler Library in Rotterdam, maintained by UEA.
• The Montagu Butler Library, maintained by the Esperanto Association of Britain.

The collection of The National Library of Austria has an online search which can be accessed here.