Been looking for walnut material myself as well, as i would love to test it out, but canīt to locate any supplier here.
Iīve generally used blasting glassbeads in my cabinet.
I find that glass does a very nice job, leaving a smooth matte surface behind.
Itīs lighter than standard blasting sand, doesnīt stretch thin sheetmetal as easily as sand has a tendency to do, while still being capable of removing quite severe layers of rust and paint if necessary.