As above ^
Also check eBay.
What your are looking for as far as holding caps is a "cap pouch". Do a search on eBay for "Civil War cap pouch".
A lot of CAS shooters use C & B - search on eBay for "CAS ammo pouch".
There are a wide variety of belt pouches out there . . . you just have to google of the right thing. If your are looking for a military style pouch - you might consider a "Civil War carbine cartridge box". They are not as "tall" as an infantry cartridge box. Most reproductions come with a wood block insert drilled for cartridges. The wood block could be removed and I believe you'd have room enough to carry a flask and a small bag of balls, etc. All depends on what your flask is.
You might also consider just getting a canvas or linen "haversack". These can also be found on eBay. I use an older "tarred haversack" that I've had for years to carry my flask, supplies, etc. when I go to shoot C & B.
Google for "Civil War Sutlers" - most carry pouches, etc. You might also go to the N-SSA site (North-South Skirmish Association) and look for their links to dealers. There are a number of sutlers listed there.
Or . . . . you could make your own!
Good luck in your search - hope you find what you're looking for.