Getting references after a company closes after 3 years as a janitorial supervisor

The company closed and I am not sure how to reach former colleagues. Any experience at all would help. A colleague offered to vouch for me. How do you ask politely?

A client reference worked well for me. It shows you deliver outside the company too.

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.