I'm going to have a full set of CB400f carbs for sale soon but they aren't gonna be the cheap option, as they are completely cleaned, polished, and rebuilt and fully operational. Currently, they are on my bike, but I'm having another set modified that I'm going to be using soon.
I think your best option is a junkyard or ebay. A good tip is to look for CB350f carbs, as the bodies are the same and demand is a bit lower. there are a few sets of each on Ebay right now.