Alty:
Have your tried DHS Zinc SHampoo? DHS Zinc Shampoo Reviews at DermStore.ca
I've been using it for 30 years. Recommended to me by a dermatologist. Now I usually have to special order it. It's so concentrated, that all you need it a drop or so.
Usually dandruff comes from oily skin. If so, then this mix as a rinse will strip all the yucky oils: 1/3 Mouthwash (Cepecol), 1/3 Witch Hazel and 1/3 warm water.
For me, I premix the first two ingredients 1:1. Then mix 2/3 of this mix with 1/3 warm water. I just use the cap of the mouthwash container. It's used like a cream rinse. Once the oily hair is in control again, I don't use this odd stuff for quite a long time.
I use the DHS Zinc Shampoo daily. If I shampoo my hair a second time I use a fragrance shampoo. When it gets out of hand, the mix.
Used to have a bad case of dandruff in my teens. This concoction fixes it.
Note: I searched for it in Canada for you.