Matthew Poole and continuators on Numbers 16:18

18 And they took every man his censer , and put fire in them , and laid incense thereon, and stood in the door of the tabernacle of the congregation, with Moses and Aaron.

Which they could easily make in a sleight manner, which would suffice for the present purpose.

Taken from the Altar which stood in that place, Levit. 1. 3, 5. for Aaron might not use other fire, Levit. 10. 1. And it is likely the remembrance of the death of Nadab and Abihu deterred them from offering any strange fire.