Hi jedwards,
Welcome to the Mattress Forum! 
I’m sorry, but I don’t have a listing of what store would actually stock true box springs, either in San Diego or elsewhere. I don’t keep a record of the individual mattresses or their specs that the retailers and manufacturers in the hundreds of forum lists throughout the forum carry on their floor or have available online (it would be a bigger job than anyone could keep up with in a constantly changing market), so checking their websites and making some preliminary phone calls to the retailers/manufacturers that are on the local or online lists is always a good idea before you decide on which retailers or manufacturers you wish to deal with anyway. When calling your local retailers, be sure to explain that you are looking for true “old school” coil box springs, as the term “box spring” is still used interchangeably by many retailers to mean any piece placed beneath the mattress.
I think that post #2 here was the box spring post to which you originally referred.
You may already have checked out this list, but subject to confirming that any retailer or manufacturer on the list you wish to deal with has an actual coil box spring … the better options or possibilities I’m aware of in and around the San Diego/Escondido areas are listed in post #2 here.
Good luck!
Phoenix