Getting references after a company closes - any advice?

The company closed and I am not sure how to reach former colleagues. I have read a lot of threads here and could not find my exact situation. A colleague offered to vouch for me. Is a client acceptable?

Use a professional network to find a former manager. Many will still write a note.

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.