It seems small can never be small enough in miniatures.
The smallest seed beads that you can find at most bead and craft retail stores are 15/0 Delica seed beads.
Anything 16/0 and higher (the larger the number, the smaller the bead) are considered "antique" or "vintage" as they are not commercially made anymore.
Here are two online sources if you need really, really, need tiny seed beads for your miniature projects.
Empyrean Beads (antique Italian seed beads 16/0 to 22/0)
Bead Cats (seed beads 16/0 to 24/0)
Note that the color selection is greatest for 16/0 and minimal for 22/0 to 24/0.
I've been told that sometimes you can find antique seed beads at estate sales, but you have to know what you are looking at.