A massive source of single use plastics are shampoo and conditioner bottles. You can replace these with shampoo and conditioner bars.
A word of warning though, it may take some time for your hair to transition to a bar. This is nothing to do with the bar but, how often does your hairdresser tell you that you should be using shampoo and conditioner free from chemicals and sulphates. Most of the products you can buy in the supermarket or Boots are full of chemicals which you hair and scalp have got used to. If you are going to rid your hair and scalp of these chemicals, it will take a little time and patience but I promise that the switch is SO worth it.
There are many companies out there who make shampoo bars but nearly all of the people in the Facebook group recommend Rush Organics, and even my husband has made the switch and LOVES it!
Other recommended shampoo bars are Lush and White Light Candles with Karey.