Reetha powder can be replace by Shikakai Powder. Shikakai powder is even milder than reetha powder so you may have to add more of it to get enough cleansing property. Sometimes you might find that Reetha powder is called 'Aritha' powder by some brands, so do watch out for that! Also, you can use soap nuts to grind up your own Reetha powder, if you are really struggling to find any of the options mentioned above!