The county would have all the info on the most recent sale which would be the Sheriffi sale, but that wouldn't be the outstanding balance, it'll just be the balance at the time of the sale. Often times the county tax regards will have a contact name listed under owner or tax payer, not just Fannie Mae. And as Danaus pointed out if you can find the attorney who handled the forclosure they can help you with a name.
But, what you really need is just some place to submit an offer too. It doesn't matter what's owed, what is it worth? If it's worth less than what's owed then they'll just have to accept less.