-ity: Popularity (the state of being popular)
-ize: Popularize (to make something popular)
-ly: Popularly (in a popular manner)
-ism: While not directly related to "popular," populism refers to a political approach that appeals to ordinary people.
-ization: Popularization (the act of making something popular)
You can also use prefixes to create words related to "popular":
Un-popular: Not popular
Over-popular: Extremely popular
Super-popular: Very popular
The best choice depends on the specific meaning you want to convey.