Many of the macrobiotic recipes call for a heat diffuser. They can be purchased at Amazon, or any store well stocked with kitchen and cooking items. Here’s a link to a google search for heat diffusers for sale: Shop for Heat Diffuser.
If you don’t have one, you can make do with a pie plate and a stove top. In the following video, Kenzi Wilbur shows you how to hack a heat diffuser with a pie plate. By inverting your pie plate over the stove, you can create an even and thorough cooking surface that can also take the place of your double-boiler.