Talk to Dermatologist on Severe Dandruff

I have severe dandruff on my head and I have been using conaz(ketoconazole) lotion for the last 5 years but it seems like it is less effective now. it is not the typical dandruff whenever i do not use conaz for 3 to 4 days my scalp becomes red and itchy and dandruff increases a lot. Which shampoo or medicine should I use to treat it kindly help someone who has recommended c-tar shampoo but it is not available in my city though it is available on some online websites. also kindly guide me to credible websites where I can buy it.