Unfortunately "How to find a rightsholder?" is not a legal question. Even more unfortunately it is not a trivial undertaking.
It is unclear at law if copyright can be abandoned the way personal property can be. It is also unclear if an item under copyright can be placed in the public domain other than by expiry of the copyright term: attempts to do so probably create a non-revocable, royalty-free licence to everyone instead.
In light of that, unless copyright has expired somebody owns it. Given that copyrights can be transferred like any other personal property (e.g. sold, gifted, bequeathed, distributed to creditors in bankruptcy etc.) and such transfers are not recorded, working out who that somebody is is problematic.
If you wish to make use of copyright material in a way that is not fair use/dealing the onus is on you to make all reasonable efforts to find and obtain the permission of the copyright holder.
If you were to diligently search for the holder (e.g. for a book by contacting the publisher and the author and following up whatever leads they give you) and document that search then you could use that documentation to protect you from damages claims if the actual copyright holder emerged and sued you. You would still be required to stop using their copyright material even if you are shielded from damages. Of course, they would need to prove that they were the copyright holder by having evidence of a chain of transfers from them back to the original holder.