Most good (5+ slot) bags are player made. For the bank, you can make (or buy) strongboxes. I currently have all my char inventory slots filled with 6-slot 10% WR bags, and have almost all my bank slots filled with 6-slot elm strongboxes.
I don't know how many slots the higher level strongboxes will have. Someone told me they heard that the highest level strongbox will be a 40-slot box, but don't know this for sure. I do know that I won't be making those, as I'm going to be an armorer.
If anyone needs bags, and you're on the Kithicor server, send me a tell. I'll be happy to make you some bags - for a price