Do you mean the terracotta flower pots? I bought mine at Dollarama (3 for 1$). Wherever you get them, make sure the drainage hole at the bottom is small enough so he cannot swim through and get stuck or cut!
A terra cotta pot? You can get em at gardening stores, or even craft stores sometimes! I would soak it for a day or so in another receptacle, just to get it fully sodden (they bubble a TON) and to make sure it doesn't leach anything. Some fish have a tendency to try to swim through the drainage hole in the bottom/back, you can either jam it against something, or fill it in with silicone gel, or even shove a rock in there or something.
I have a small one that came with some seeds and stuff. It never grew so I just rinsed it very well and placed it in water for a few days. I do believe that one came from China, and it's been fine in my tank