The following nursery seems to have it:

Here is a link that might be useful: Henna

FWIW, I recently got a henna plant, read up about care, then nearly killed it.

The websites all said it's a desert plant and likes it hot and dry. So I put it in my greenhouse and didn't water it.

It nearly died of thirst. So I took it out and put it where it would get a little rain (we get 170" a year here).

Again, wilt city. So now I have it next to my pond, and when it looks thirsty I dunk it. Much happier.