Best way to obtain Black Powder shells is to load your own. Simply not enough demand for them to be loaded at an economical price commercially. depending upon the quantity needed they can be loaded very simply with little if any investments in tooling or if a higher volume with regular loading tools with a bit of attention to the powder measuring. It is best to use either paper or all brass hulls.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra