Hi scubamom,
Done 
[quote]For a temporary fix, wonder if we could shove two bungee boards under there?
There is a local mattress company (Cantwell) who still makes Latex beds (we still have two of them - 40 years old and still going strong) - I might be able to buy whatever foundation they use for their latex beds.
On the other hand, if it is a combination of the foundation AND the mattress… ??? [/quote]
If you can I would suggest sleeping on the mattress on the floor for a couple of nights to see whether that resolves the issues you are having. If it does that would confirm that it’s your foundation.
You could add a bunkie board (see here) or even plywood on top of your foundation to create a more evenly supportive surface (although I would probably cover it with fabric to reduce the chance of sharp edges or splinters harming your mattress). While I’m not a fan of solid support surfaces (see here) … for the short term or if there aren’t any other risk factors for humidity it would probably be fine.
Assuming you’ve confirmed you foundation is the issue I’m not sure what PLB would be able to do even if you were to reach them unless your foundation is defective or they have introduced a new foundation and were willing to replace it but replacing it with a suitable foundation from Cantwell (or any local store that sells a suitable foundation) or with one of the foundations that are listed in the foundation post here would probably be a good idea in the longer term.
Phoenix