You can buy a scroll case and a potion case in Waukeen's Promenade but you won't find a bag of holding until Spellhold (you can however get something very similar if you recruit Hexxat into your party).
Easiest way is to get Hexxat and do her first quest. It is not that hard, though +2 weapons and/or some damage spells (other than cold) are recommended.
Or enable cheats and clua console a bag of holding into your inventory. It is a cheat, but if it makes the game more enjoyable to you it is okay.
i recommend people to rely more on the item-specific containers. bag of holding, if you use it indiscriminately, creates a mess
the more specific containers can be consoled in at any time. it's not cheating unless you have a weight problem and use the potion case to reduce the load. money is not important. same goes for bag of holding, but creating it might just not feel right for some people because it's a rare and magical item...also, before late game and tob, you won't have so many items such as weapons you don't use but might want to keep to need to use a bag of holding to free up space
Keep in mind for those who cheat if you have a bag of holding with stuff in it and then get the one underneath Spellhold, the new one will have all the same items in it that the old one does. Nasty bug that will likely never be fixed.
That's not a bug but rather the way how bags and other containers are implemented in the game engine. You'll most likely never encounter this effect unless you cheat or use mods that add several containers of the same kind.
Also it's not much of a problem because they are really two copies of the same bag (they point to the same store file to be more precise.) If you take an item out of one of them, it will disappear from the other.
Comments
Or enable cheats and clua console a bag of holding into your inventory. It is a cheat, but if it makes the game more enjoyable to you it is okay.
the more specific containers can be consoled in at any time. it's not cheating unless you have a weight problem and use the potion case to reduce the load. money is not important. same goes for bag of holding, but creating it might just not feel right for some people because it's a rare and magical item...also, before late game and tob, you won't have so many items such as weapons you don't use but might want to keep to need to use a bag of holding to free up space
C:CreateItem("bag31")
I still buy the other containers ASAP.
In a party, I let the rest of the party be my pack mules.