Clean substitute for black powder

I have purchased a black powder pistol in .38 s+w. But there’s no ammo to be found online, I could pull the bullets from modern ammo and replace with black powder.

However I have there is powder that has the same safety-related characteristics that I want but do not yield clouds of Stygian fumes which might attract unwanted attention at the range. What is my best choice for this, and where can I get it. Google searches have yielded nothing since apparently I don’t know what it’s called

Thanks!