Hi elapid,
I don’t know of a manufacturer that still makes box springs and sells them online. There are a fair number of local manufacturers that still make them and if there is one in your area they may be your best bet. There are a few examples of manufacturers that still make box springs in the Massachusetts area listed in post #2 here and I know there are many more across the country but I don’t have a specific list of manufacturers that still make them.
Phoenix