Answer: Unfortunately, I am not certain which plant you are looking for -- although in a brief search I discovered several plants that are new to me. Perhaps it is one of these below.
There are several types of Australian native plant called Philotheca (now including most of the Eriostemon as well), sometimes called wax flower, that might meet your description, however the flowers are more usually pink. I have found references to white flowers on it, but no yellow.
There is also the Geraldton wax flower or Chamelaucium uncinatum, another Australian native, however it has flowers in the pink/purple range and I have not found any reference to a yellow. Here is a photo of one.
And, although the plant is nowhere close to your description of it, Hoya carnosa is often called Wax Plant, too.
I did find mention of a naturally occurring hybrid between the Chaelaucium and Verticordia which might allow for yellow blooms -- at the end of the following article.
I'm sorry I can't tell you where to find the yellow wax flower, but I hope this helps you get started in your search. You might check with your local county extension and reference librarian to see if you can identify the plant. Once you know that, we may be able to locate a source.
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