Assuming you mean a Pterygoplichthys gibbiceps, I find that quite surprising. In the UK you can barely give them away. I would try some classifieds, people are always trying to re-home them.
His problem is he wants a decent size one. All the ones the wholesalers will stock are farm raised 2" babies that people will be buying to stick in their 20gal tanks.
What you need to do is find one that's been traded in, or adopt one from a 20gal tank owner, once it's grown to 6".
Or buy a 2" one, stick it in a 20gal with a heavy duty filter and lots of food, and wait 3 months.
I'm assuming a baby one will be eaten in your tank, and that's why you want a bigger specimen.