Who do I ask for a reference after a layoff? - what worked for you?

The company closed and I am not sure how to reach former colleagues. There are 12 people counting on me, so I am trying to be careful. HR only confirms titles and dates. Is a client acceptable?

Thanks for writing this out. I went through something close to it 2 months ago, and it settled down once I kept a simple log of every call and form.