Hi Condit,
They are certainly still available … but the trick is finding a manufacturer that uses them (or makes them available).
Leggett & Platt supplies the majority of springs used in the industry and they make one called WebLok which is the type you are referring to. They also have what is called a Torsion bar system called True-Flex which also has “spring” to it which you can see here and which is more commonly used.
There are a lot of smaller local or custom manufacturers that still make them but unfortunately none of these are particularly close to you. In the more widely distributed brands … a few that still have them include Gold Bond (which are probably available close to you), and Eastman House. Their sister company Eclipse also has them available and Therapedic can probably make one for you as well. Englander generally uses a Torsion bar system but the local licensee will make a coil box spring on request (this has to go through a store). There are also some more premium brands that make box springs with coils such as Aireloom but you are looking at much higher prices with these more premium manufacturers.
Some of the choices in post #4 here also either make them or will special order them (including Verlo which will make them as a special order).
In the other direction post #2 here may have some options and I know that Winndom will make one for you.
There are probably lots more but these are the ones that I know of or could find with some quick searching and a few phone calls and at least give you some “true box spring” choices that are within reasonable driving distance.
Phoenix