Yup. And considering hobbits were making their own fireworks, it's more than plausible that Saruman got his hands on them through the same channels, explosives tend to draw interest in a world without them, so he may very well have gotten his hands on some the same way. He may also just have already known how to make gunpowder like gandalf did. He was a servant of Aule before being sent to middle earth. I like the hobbit explanation though cause it fits really well thematically and characteristically. Saruman was a good manipulator and good at using other people's tools to his own ends but tended to shit the bed at creating his own stuff.