Jason the marketing strategy has changed over the last 10-15 years as to buying versus leasing. Even if you could find a DOS version (5.8 may have been the last) I am not sure it would run on XP or later. Windows 98 was the last main OS supporting DOS very well the best I remember. Next you are going to have to convince someone at Xactimate to activate the DOS version but I doubt that is an option from even a technical stand point. I am certain PowerClaim 98 and it was Windows 95 based would not even install on a computer today. If it did we would have not way to activate it because the hard drive containing that software crashed several months ago thankfully.
Yes you could buy an old computer with 5.8 running on it but just moving a computer that old could kill the hard drive. If Xactimate is of any cash value to your business you would be financially better off to get a usable solution perhaps but then you may be planning to close shop in a couple years.