A simple web search under "Lost Life Insurance" will offer many fee-based services. These private companies generally fall into two categories: a) those that send out email/fax/letters to all life insurance companies on your behalf, and b) investigative asset search services that get paid based upon a percentage of any assets recovered. Unlike other fee-based services, MIB's Policy Locator Service is a unique industry database of application activity that may be helpful in your search.
MIB Solution's Policy Locator Service (PLS) may provide evidence of life insurance applications taken by MIB's 430 member companies on a decedent.