With the years I know my traders and send messages, some weeks before I visit them. Some types of beads became much rarer over the years. The "typical" 6-layer chevron for example are nearly gone...
Beads are often not displayed in the shops, but sometimes, when you ask for, they can show you plastic bags full with beads...But this is only for "Tiznit" and Marrakech, maybe Essaouira as well. I donīt know about the places more in the north...