welcome to the fascinating addiction .. bye bye money - don't forget to write ...
patches and such can be had at Track of the Wolf, but i avoid the pre- lubed stuff because you don't know how long they've been laying around ( having said that, Track probably has pretty good turn over, so you're probably safe) ... Dutch Schoultz' method is really good - the best non-shooting accessory you can buy, to my mind. If you follow his instructions, your groups will shrink: he guarantees it.
If you can get real black powder locally, by all means do so! It supports the merchant (who has to jump through all sorts of BATF hoops and really doesn't make much (if any) profit on the deal- i have to drive about 45 minutes one way to buy BP, or pay the HazMat charge... grrrrrr ... don't use anything other that black powder or Pyrodex in your rifle. I worked for an attorney who got a decent start by taking a case where the newbie was told that "it's all black" by the sales clerk ... ta da!!! instant pipe bomb... even though there were warnings all over the barrel, and in the instruction booklet (which the plaintiff freely admitted he never read) the jury found his way ... this proves that juries aren't all that smart, but that's a tirade for another day.
When you go to clean your barrel, as described earlier, just un hook it, plonk it in warm (not hot) water, and run a patch up and down - the suction will draw the water in and you'll get an impressive cloud of funk. change the water. Do this until the water runs clear and you're all set. Then pick a rust inhibitor. Some folks run isopropyl alcohol as a final swab before the oil. If you want to start a fight, tell someone that their secret rust preventer is no good and they should try your secret formula ... no -don't - just go down into the Cotton Club and drop the N-word a few time, that's safer ... I like Ballistol, but go with what works for you. You do want to be really something of a fuss budget about keeping the bore spotless.
Any one who tells you that their magic gadget makes cleaning unnecessary is trying to separate you from your hard earned, over taxed, God- entrusted dollars, and when you use their junk and your bore turns from a nice shiny tube to a rusted sewer pipe, they won't be around. Get a good cleaning jag and use it often. Dutch Schoultz' method tells you all about this.
powder measures can be had just about anywhere. i've used one like this for about fifty years:
https://www.trackofthewolf.com/Categories/PartDetail.aspx/85/1/MAS-120eventually, you might become interested in making your own horn, and then you'll have a "most accurate" load for your rifle, so a single volume measure will do fine. Until then, go with the adjustable.
I'm not a fan of scopes on muzzle loaders, but it's your rifle. I believe that if you have reasonable visual acuity (i.e. if you're safe to drive an automobile) you should be able to get groups tight enough to drop Bambi out to about 80 yards, so a scope is another gadget to break or malfunction at exactly the wrong time. But that's just me - do what works for you.
As regards "sabot" and all its works: humbug ... what you're shooting here is a muzzle loader, not a WinMag 300 macho-man CF rifle. If you want to turn Bambi into little white packages at two klicks, go with the WinMag. But for the traditional hunting, a patched round ball will work fine, and you need not succumb to magnumitis and try to get the PRB going a zillion feet per second: a 'rule of thumb' is one and a half times the caliber for a good 'starting load.' Thus, if you're shooting 50 cal, go with 75 grains of powder, and see where your groups hit, then back it down five grains and see where it goes... follow Dutch's instructions.
You'll do fine. Practice. Have fun and, as always, Make Good Smoke