In the first instance, we would recommend submitting your request to the publisher. If rights have reverted to the author you will be notified.
We have a partnership with ALCS (the Authors Licensing and Collecting Agency), who will search their author databases on your behalf. We also recommend you search the Watch File, which is a database of copyright contacts for writers and artists.
In some cases, it may not be clear who the current rightsholders are and we may have to direct you to additional permissions resources.