It may be possible to find the values you are looking for in the documents associated with the deceased by contacting the executor, or whomever handled the estate at that time. There might have been an appraisal (or some sort of valuation determined) for the DOD value of the property, or for the distribution/disposal value for when the assets were distributed to the heirs. When the property was sold, there might/should have been an appraisal done on the property for the sale/purchase transaction which you might be able to obtain from the seller or buyer or someone directly associated with the transaction, if an appraisal was done.
If an appraisal was done when the property was sold, and you can get a copy of it, I would highly recommend you have it thoroughly reviewed for accuracy. In most areas, the height of the real estate "boom" was in 2006 as was the start of the decline. Many markets were exposed to a variety of fraud acts which included inflated values of real estate, which could be a factor in determining if estate taxes might be due or not, and if there is any due it could be a factor in the amount owed.
I would ask a tax advisor or attorney as the county tax appraisal info is public per county, but the info you need is really tax advise. We are licensed to practice real estate, but not real esatate tax advice and not real estate law. Sorry!! :)
Here is an attorney who
could help: http://www.marchandmoraine.com
Our MLS system doesn't keep listings that long. May be your CPA can tell you if you can get away with the tax assessed value that year. I can provide that for you.