Wills, probate, letters of administration, powers of attorney
You will need a death certificate and a copy of his will and sufficient documentation to prove a link to the claimants. You do not need to use the services of any organisation who claim to locate the missing funds. If you do use such services they will claim a significant commission to find the funds.
Be aware that often the amount of money in such unpaid accounts is very small, so don't spend it in advance.