@simrin-- You can find McCormick and Spice Island brand Saigon cinnamon in grocery stores. Watkins also carries it and that's a very good brand that's available for purchase online.
The quality is different for different brands. You might want to check the amount of cinnamon oil in the Saigon cinnamon you buy. The higher the amount of cinnamon oil, the better quality it is. I use Saigon cinnamon with 5% cinnamon oil.