Worm castings should be as fresh as possible. That should explain why the consensus is that any commercial product is going to be of less quality than a home made product.
One option could be to search the internet for a local worm guy, and try to get some fresh castings for immediate use. This would likely be superior to any 'commercial' product from any garden store.
A bag of castings/vermicompost cannot sit on a shelf, or travel around the country without losing quality. This is my experience but also what you'll read here on the forum. The science to back it up should be that the bacteria/archaea (or other life) won't survive without moisture and oxygen.
Remember, a quality product should be full of life, and teaming with microbes