There are several options of shops where you can purchase an inflatable bathtub. Options suitable for babies and children can be found at children's shops and department stores like Americanas, Carrefour and Magazine Luiza.
Larger models for adults, on the other hand, are easily seen in shops that sell construction products like Leroy Merlin. In online shops you can also make this purchase and some options for this are Amazon, Submarino and Shoptime.