Getting references after a company closes after 6 years as an office manager

The company closed and I am not sure how to reach former colleagues. I have been trying to stay organised about it. HR only confirms titles and dates. Is a client acceptable?

Thanks for writing this out. I went through something close to it last year, and it got easier once I kept a simple log of every call and form.

I kept a note of my results so my references could be specific. They appreciated it.