The difference between using leather for storage/carrying, & actually using to load your muzzle-loader is that the one you use for loading needs to be safe to use. Flasks & powder horns have a safety feature, a pressure release. On metal flasks it is lightly soldered seams, on a powder horn it is the base plug. The base plug should NEVER be glued, only sealed with beeswax & secured with a few short pins around the perimeter.
I don’t recommend making a leather powder flask, the nature of a leather flask would require you to carry it spout upward. IF a spark were ever to ignite the powder in your flask, the pressure of the explosion would be outward towards your body, & upward towards your head/face.
Horns can be found at abattoirs if you have any nearby. You need to find a cow horn, NOT a bull horn, they are different inside.
Another source is purchasing an inexpensive ready made powder horn. Mostly these are NOT made correctly, often the base plug is not sealed which means fine powder can leak out & this is dangerous, also of course it will not be waterproof. You can either: 1) try & seal the base plug without removing it using melted beeswax 2) Remove the base plug, seal it & replace securing pins or make a new base plug 3) cut the horn & the base plug off & fit a new base plug.
If you use a pin punch you can drive the pins further into the base plug to remove the plug. If the base plug has been glued in, then you will probably have to cut it off, or drill a series of hole in the plug until you can remove it bit by bit.
DON’T fit a brass measure to your horn, NEVER load directly from your horn: https://woodsrunnersdiary.blogspot.com/2011/04/correct-way-to-load-flintlock-gun.html
Making A Powder Horn: https://woodsrunnersdiary.blogspot.com/2018/12/making-powder-horn_31.html
These powder horn suppliers are in the US, but their prices are probably less expensive than here in Australia. Get the largest & least expensive horn available.
Any questions Ob, just ask.