My mom recently died and we are unable to locate who she had life insurance with

Search all of her old bank statements, it may be that she was paying with a check, monthly or maybe on a draft by the year. If you find any name of an insurance company, call or write to them if her insurance was with them they are bound by law to tell you.

This is a common and seroius problem,

You will need to go though every paper, every letter she has filed anywhere. Go though every check receipt you can find, old check books. Also get all of her mail forwarded to you or someone and look for notices that may come in over the next year.