Where do growers get seed for seedless watermelon if the melons have no seeds?
Good question. It's a complicated process that involves crossing two diploid melons to get a triploid. At bit like crossing a donkey and horse to get a mule.
And like a mule a seedless melon is unable to reproduce so in order to get another seedless melon you need to repeat the process.
Here is an article that explains the process more in-depth from Michigan State University Extension: Seedless watermelon – how do they do that?
Thanks for your interesting question.