That seems backwards IMO; you need their permission, so there's no point in finding the code if you can't get permission, and the copyright holder is more likely than almost anyone else to know where to get the source anyways.
There's no point getting permission - via a potentially involved negotiation and lawyers and contracts - if you can't even find the code to fulfill that contract with.
In the muddle of defunct companies, mergers, lapsed contracts, etc. - figuring out who actually owns the copyright can be a bit muddled. An engineer who may have squirreled away a copy - or who knows where such a copy might be squirreled away - is more likely than almost anyone else to know what company might own the copyright, and possible points of contact within that company to contact to even try to begin negotiation, even if they no longer work for the company.
Let them find you. Ask for forgiveness, not permission. It's likely no-one gives a fuck about the copyright any longer, so it's unlikely to have anyone knocking on the door. Whoever owns the IP probably doesn't even know they own it.
