Severe Dandruff Shampoo

So I'm currently trying to figure out which shampoo to get to eliminate my very severe dandruff. I've heard good things about Selsun Blue but I want to make sure I get the 2.5% selenium sulphide version and wasn't sure if Japan sells it (Amazon jp doesn't specify the percentage). More specifically, do I need to get a prescription for the Selsun Blue 2.5% in Japan? In the US unfortunately it's prohibitively expensive to get a prescription so I'm hoping the kokumin nenkin can help. Currently, I have to buy a ton of Nizoral 1% Ketoconazol 400 ml from the US and have it sent over via family to Japan, which isn't ideal and very expensive. Also, Nizoral doesn't even really work for me since I have to use it 1-2 times a day just to notice anything. Furthermore if there are better alternatives, let me know (i.e. Nizoral 2% Ketoconazol) and please send links if available. Thank you in advance!

EDIT: I found Selsun Blue Gold 2.5% Selenium Sulfide on Amazon JP! Turns out it's made in Australia and I just had to do some basic math to get the percentage. For ¥5600 ($36 USD) it's the same price as Nizoral 1% Ketoconazol in the US. Thank you for all the help!

Here's the math:
25mg/ml Selenium Sulfide

200 ml Bottle

25 mg/ml * 200 ml = 5000 mg = 5 g of Selenium Sulfide for the entire bottle

Water = 1 kg/liter = 1000 g/liter (this is density)

Assuming the both densities of:

the bottle's substance = water

This means the 200 ml bottle's contents roughly weight 200 g

5 g Selenium Sulfide/200 g Bottle Contents= 0.025= 2.5%

I.e. 2.5 % of the bottle is Selenium Sulfide 😀