Place a poppy in a crafting grid to get 1 red dye
Place a red tulip in a crafting grid to get 1 red dye
Place a rose bush in a crafting grid to get 2 red dye
Place a beetroot in a crafting grid to get 1 red dye
Place a poppy in a composter to produce bone meal, not red dye
Use a wandering trader to obtain red flowers, then craft them into red dye