I know it's shocking, haha.
You should visit some famous fruit stores, like:
http://takano-online.jp/category/FRUIT/
https://ginza-sembikiya.jp/category/mellon
http://www2.enekoshop.jp/shop/sunfruits/item_list?category_id=452644
You might find a square shaped watermelon!
I will visit these stores, thank you so much for the links!
Oh My - A melon for 16,200 Yen ($150), it's unbelievable!