Getting references after a company closes after 13 years as an insurance claims processor

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

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.

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