Care and feeding of a muzzleloader can be an expensive or cheap proposition depending on your mindset.
In the anxious wait to actually shoot your new acquisition, it is too slow to actually make some of the accessories. For many of us long time shooters, part of the satisfaction comes from doing and making our own. It is easy to spend as much as the gun on accessories. Then again, there are the following ccategoies:
a: most absolutely essential = powder, patches, balls, caps, ball starter, powder measure, horn or flask
b: required for care of the gun = proper cleaning jag, ball puller, cleaning patches,
c: real handy to have nearby = pliers, nipple wrench, small oil vial, patch puller, range rod, extra nipple, screwdriver, extra barrel wedge,
d: really nice to have = possible bag, CO2 ball discharger, small vial of grease, cleaning solvent, small file, tackle box for all of the above.
e: sometimes used: nipple pick, different size balls, different thickness of patches, small bottle of cleaning patch solvent/cleaner (moose milk, etc)
scissors, small knife, small piece of scrap carpet to protect the butt plate at the range,
I've probably forgotten somethings.
You can either make or scrounge up nearly everything on the list. A ball mold costs around $15 to 20 and you can make your own projectiles for the cost of 2 boxes of commercial stuff. Bullet starters, ball boards, possible bags, etc are all easy to make. You can buy cloth at a fabric store for patches for about 1/10th the cost of commercial stuff. Old sheets and other items make good cleaning patches. (wash the cloth before cutting and use.)