There is a dive shop at the Christiansted waterfront. Dive Experience -https://divexp.com/. They always seem busy anytime I've walked by.

I see a couple advertised near the Fredriksted waterfront. Cane Bay Dive Shop - https://www.canebayscuba.com/ and Adventures in Diving STX - https://www.adventuresindivingstx.com/. Give them a call regarding tank refills.