I'm assuming the $15,000.00 he promised you was in the form of insurance proceeds. If you cannot find the paperwork showing the carrier and account, there are a few places you can start to look.
You may want to check with his bank records to see if there is evidence of the purchase there. Specifically, there may be a check or checks to the insurance company which will give you the name and, possibly, account number if he put it in the memo.
His records may also contain he original or a copy of the insurance agreement which, of course, would have all of the relevant information.
It's possible that your father's work or union provided the insurance coverage (even if he was retired or otherwise not working at the time of his death). You may want to contact prior employers and/or unions to see if the policy was purchased through any of them.
If your father had a will or an estate was otherwise opened, you may be able to speak to the executor/administrator who may have information on the policy.
Hope this helps.
/s Donald Kudler