In order to get chonkers, you need a food bowl. Without one, it is impossible to get a chonker. The standard food bowl is relatively cheap and has a 2% chance of getting a chonker when a cat reaches level 1. In order to get the money for it, you can go to the settings and change the distractions to "Intense" and you click on the petals. This should get you enough money. If you don't want to lose your streak as often, turn on "chill mode" in the settings.

In order to get more chonkers, you can use a roll multiple times. You can sell all your low income cats and replace them. You can even replace the cat with one the exact same breed. The more you roll, the more chonkers you get.

Note: If a cat is already an adult, it will not become a chonker. This means that if the cat is at level one or higher and is not a chonker, it will not become one after.
Another thing: There is no such thing as kitten chonkers.