I am not sure exactly as of what month, but I know that since around November 2019, the D21 and D10 have been OOP, and stocks were dwindling. So unless you have a supply of them or know someone you can get them from, it?s unlikely you can get them anymore.
There are some supplies of D21-4, D10-5, and D10-3 left. Check out Buyrocketmotors, Apogee Components, and Rocketryworks. Rocketryworks will have the highest overall stock and a competitive price.