Best not pour from a flask, if an ember is down bore (rare) the flask could explode.
I pour into a measure then down the barrel.
For the .45 Kentucky I use a cut down 444 marlin case as a measure tied to the bag.
For my larger guns I use home made antler measures. I have used just cut down shot shells before now.
I do sometimes make pre measured paper cartridges yes. It speeds up loading a little. Here is an old photo.
Percussion caps are widely available yes.
U.:-)