I think what Mike means is "heavy duty" rather than commercial chassis; something that has not been built by Cadillac since 1984. The chassis for your hearse, Jim, is a 1992 Cadillac Brougham with the 5.7 liter engine. This means that the base chassis (before any cutting by Eagle) was heavy duty, including springs, shocks, cooling, trailer-tow package, etc.
You have asked me about technical manuals for Eagle coach parts, of which I have none. Your car is pretty simple: The only mechanical things Eagle really altered were the driveshaft and wiring harnesses. The chassis itself was already about as heavy duty as it was going to get.
As Mike mentioned, just look for Cadillac Brougham parts via a parts house. The only question you should have to answer is if the car has a 5.0 engine or 5.7, and you know the answer to that.