Clowns have a shorter incubation than most, so people are leery of sending them by mail. If the mail is delayed, the breeder is the one who gets called names. It isn't usually worth it for common species of killie.
The less common ones tend to be attractive to people who have been to the dance before, and know what can go wrong.
A word of advice - importations to the US need a permit, and the permit is expensive. You can import almost anything, but you have to have the paper work, and if you don't, you get fined, heavily. Your best bet is to find the closest killiefish affiliate club (aka.org) and check on their sites. Out of borders and the mailing software makes it a waste of time to send to the US.