Many people seem to buy softboxes, use them for awhile and then sell them.
Others buy some, use them for a while, and then buy more.
I currently have 3 softboxes
This is the brand I bought, super easy to setup, fold and unfold, it uses a structure system like an umbrella, it has an inner diffuser and the cost is relatively low.
I have a 24x36, a 12x56 and a 36 octobox,