Substitutes don't work as well in the traditional rifles without some conversions. A musket cap nipple will help with Pyrodex in a traditional gun. 777 degrades ridiculously fast (better shoot it all in every year). I can't speak for BH209 in those guns at all.
BP is very stable and works well in all of those MLs; tradition, inline, or modern. There are several online vendors that will ship it directly to you. You can probably find a traditional shooter in your area through here or through other boards that have a greater population of traditional shooters. Not only would they be a great resource to get your going, they'd likely be a good source for BP and/or a split order on BP.
If you like the traditional guns, get one and work through it. If you're sold on an inline, go that route. If you want a modern gun, get one. Each will have it's own needs that you'll have to cater to.
For what it's worth, I shoot or have shot several in each category.
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