Talk to the owner of the fish store in your neighborhood and see what species of killies they can get for you.
Then learn how to breed them if you want a never ending supply.
If you get bored of them, then you can trade your remaining killi stock for other types of fish.
I enjoyed having killies for their insect eating abilities until I figured out they have a tendency to jump out of the factory cutouts in the back of most tanks.
Do they get along well with shrimps?