Finding the plumbite….

My sim has one more element to find and it’s the one from the plumbite and I’m having trouble finding it. She’s a scientist and I’ve asked all the scientist at work every day and I’ve even had her try to dig around for one too but still no such luck. Without cheats, what’s the best way to get one? I feel like sometimes I get them when I don’t need it but when I do need it is when I can’t find them.