I did this with my car back when I had an accident. I was lucky there were tons for sale. In your case you might not be able to find this. I assume this is in regards to an accident and you wanting to get the real world value of the car vs the book value. In that case it is in YOUR best interest to do the research and find what cars in your area are going for. I wouldn't trust the research someone else does.
With mine I found cars slightly above and below mine in mileage, with very similar options, broke them down into a nice chart, and sent that to them. It worked out, as I got $4000+ more than I owed and a bunch more than book value on it.
As for where to find these cars. Craigslist, Autotrader, special interest possibly? Not sure if one of them holds more value than the other though, and not sure if they discount it if the cars don't sell. I only needed to find what cars were being sold for, not what the final prices were.