For shooting bags (often called possibles bags) you can find them on ebay, Track of the Wolf, October Country, The Possibles Shop, etc., etc.
You will find a wide price range and anything from pretty basic to downright fancy.
From experience and reading I will say that most buy too big of a bag and put too much stuff in it and then end up buying a smaller bag and taking stuff out that they haven't ever needed in the field. I like about an 8" x 8" bag. I also like a divider in the main pouch and then a smaller pocket on the inside back of the bag, it really helps to organize and find stuff quicker.
With my percussion rifle I have a nipple pick/cap remover tool, nipple wrench, jag (which is sometimes just left on the end of the ramrod), powder flask, ball bag with only about 5 in it if deer hunting, short starter, palm saver (I'm a wimp lol), old cap tin with prelubed patches in it, and an old plastic snuff can with damp swab patches in it. I wear a capper and a homemade antler powder measure around my neck. My ramrod has a built in ball puller in it. About the only thing that might cause me trouble one day is I don't carry a patch retriever in my shooting bag, but it is in my main bag in the pickup. I've never had to use one ever so far. You can eliminate the powder measure, short starter, tin of lubed patches, ball bag, and the powder flask if you choose to carry some of the TC 4-in1 quick loads that are already prepped. Just depends on what a person's preferences are.