you should be able to order them from any decent shop if you are prepared to wait a while but be sure you can house it because you will struggle to re-home it later.
THANKS GUYS BEEN DOING ALOT OF RESEARCH ON THE CATFISH AND I KNOW HE GETS PRETTY BIG. I ALREADY HAVE A 500 GALLON POND UP AND RUNNING FOR HIM. I HAVE A 14 INCH WATER COW AND A 13 INCH CLOWN KNIFE IN THEIR, JUST TRYING TO FIND ANY SIZE ATTU.