Hi Nnmsgst,
The tutorial post includes this link to the members of the site that sell online and many of these make latex and latex hybrid mattresses with a wide range of designs, options, features, and price ranges. I don’t know specific prices off the top of my head so you would need to check their websites to see which of them meet your specific criteria.
Some of the lower budget latex and latex hybrid mattresses (under $1000 with mattress only in queen size) are also listed in posts #3 and #4 here but this wouldn’t include all the options in your budget range which is a little higher.
The better options and possibilities I’m aware of in the Minneapolis / St Paul area are listed in post #2 here.
I forgot to mention in my last reply that this may not be a good idea. If you have a box spring that has springs inside it then most foam mattresses (polyfoam, memory foam, latex foam) will generally do best with a rigid non flexing foundation rather than a box spring. There is more about the types of support systems that are generally suitable for different types of mattresses in the foundation post here (and the two links in the second paragraph). If you do decide to re-use your foundation (which would probably be risky after 12 years with higher weights sleeping on the mattress) then I would test it very carefully to make sure that there are no soft spots or sagging (see post #2 here) and I would also make sure that your box spring wouldn’t void the warranty for the mattress you decide to purchase.
Phoenix