Getting references after a company closes - any advice?

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. Is a client acceptable?

Thanks for writing this out. I went through something close to it 11 months ago, and it settled down 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.