Welcome back. And Welcome to the forum. You need to do some exploring, and reading of the topics here to do some catching up.
The PRB and Black Powder will continue to kill deer just fine. You may have trouble finding Black Powder. Most of us are now ordering the powder shipped to our doors from one of several distributors. Graf& Sons, in Mexico, Missouri, is problably the closest one to you. Then there is Powder, Inc., in Arkansas, and Jerry's Powder keg, in Louisianna. Go to the "Links" section at the top of the index page to this forum to find a list of powder sources. Click on the names and you will get contact info, including 800 numbers to call. Delivery is very good- a couple of days at most for UPS to show up at your door. You may also want to buy percussion caps, but they are expensive, and you probably need to buy a thousand of them to justify the shipping charges. Even then its expensive.
You don't say where in Northern Illinois you are located. If you don't belong to a BP shooting club, it would be worth your effort in finding one, and joining. That way, you can pool orders for powder and caps with other shooters to reduce the cost even more. And, if there is a local retailer that carries caps, the members of the club will know who it is.
I see NO advantage to using the substitute powders, much less the sabots. They are designed for In-L*%* guns, with fast twist barrels, not the guns you have. The Flash point on the substitute powder, except Pyrodex, is much higher than that of Black Powder, requiring Magnum percussion caps, when you have been, and can continue to use standard #11 caps.
If you still have old patches, or patching material, you may want to buy some new stuff. The old stuff tends to rot or deteriorate over time, and there is no point in putting up with the frustration of having torn patches blow your groups all over the place. Its just ruins a range session, while you search out a fabric store, and buy some new fabric. Most people are shooting a .530" diameter ball with a .018-.020" thick patch in their .54s, using 80 grains of FFFg for a hunting load. If your shots are going to be closer than 75 yds, you can back that powder charge off a bit- say to 75, or even 70 grains, and the ball will still completely penetrate a deer on a broadside hit to the chest. Pick a load that provides you the best accuracy, and then place the ball where it needs to go to give you a clean, quick kill. :thumbsup: