Around here, I used to pick primaries up by the hundreds in the local parks when the geese were molting, usually around late June. Unfortunately, I don’t know of any commercial source right off the top of my head. You may want to Google “Rainbow Feathers” and see if they carry goose primaries.
Visit the nearest farmer that has geese, I’m sure he’d be more then happy to let you have some of the feathers that they dropped. If there are no farmers around, I don’t know any retailer that has them.