Where exactly are you going in Morocco? When I was in Marrakech, there are many ladies doing henna tatttoos in the main square of the Medina, Djemma El Fna. I would imagine any main square would have henna ladies.
If you do find some henna ladies in the square, just be very careful that it is real henna, smell the stuff they are using. It should be earthy smelling. Never have black henna done on your skin. If the mixture smells sharp like chemicals, it has been mixed with hair dye which can burn your skin.
I saw lots of henna powder in the souks that carry other personal products like savon noir soap and tiny pots of natural lip color etc.
Enjoy your travels!