Sleeping Sues are referenced in book two (Through the Door) of the series.
In Through the Door it’s revealed they’re available at florist shop Petaling in Kincaid.
“Now these, these are Sleeping Sues. By day, they look like any decorative plant you’d use for ground cover – think of Lamium or even Bishop’s Weed. But at night, a thick batch of blooms rise out of the leaves. The purple flowers are protective of whatever they’ve been planted to guard. If anyone should dare to cross over them, they puff out a little powder. Once inhaled, the intruder is put into a long, deep sleep. Quickly.”
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