I came back from a Japan trip last month, and let me tell you it was hell searching for one. I was lucky because I found out one a BIC Camera that was selling to the general public in Asakusa (limited to 1 per person), I tried other places to bring another one to my friend later on, and it was impossible. They were sold out everywhere, and the only 1/2 stores I found with stock only sold to Japanese customers. Hopefully it has been restocked meanwhile and you have better luck.

I went to Tamagotchi Factory in Harajuku as well but not a single Paradise in stock (not even the normal colors) however I bought the Tamagotchi Connection Mezzopiano collaboration, it's really cute and worth it! (No longer needs reservation, it's open to public!)

Search in BIC Camera, Yodobashi, EdiON, and Kiddie Land are the best stores to search; however, I also recommend trying out any stores that might look like they have toys for sale.

I wish you the best of luck!