Hi, Glo. The problem is one of availability. Current forest managers just do not let their trees grow as big today and when we were originally logging the country. With smaller trees, you get smaller boards.
You can find wood like you are seeking through architectural lumber dealers and specialty sawmills. These are the places that buy old buildings and tear them down for the wood, in the first instance. Logs were floated to the mill in many parts of this country and inevitably some of the logs sank. In places as diverse as Wisconsin and Georgia, the sunken logs are being recovered, sawed and dried so that the wood from them is available. The problem is that it is very expensive and, therefore, the wood is rare and expensive.
Specialty woodworking magazines sometimes list sources for this wood but a lot of it is sold by word of mouth. You might also just start searching the web. Another source might be a post at www.woodweb.com on the sawing and drying board. It is out there but be prepared to wait for it and pay for it. Knowing exactly what size you need will be necessary.