My thought too. Yes they grow slow but a 1.5' wide tank is too small even for a 1' sub-adult IMHO, not to mention the 2' adult.
Snookn21 on aquabid carries them almost all the time but, again, if there is no plan to upgrade, I'd stick to cats that do not grow longer than 1/2 the width of your tank. There are those who fervently disagree and profess such a catfish can be as long as their tank's wide. You make a decision.