Seeds for planting are sometimes treated with chemicals, so that might be a risky way to go. Ya know, watermelons weigh so much that maybe they are not sold mail order like other fruits are.
Are you open to another method of kidney cleansing?
The Watermelon Seed Tea recipe comes from a psychic channel who died a long time ago. I have never heard of any holistic health experts suggest this particular method for kidney cleansing... it is more suited for "Cayceites" - people who are big into his channeled messages.
I use Celery Seed Tea and parsley tea. There used to be a lot of info on how to prepare them on the homepage, but now that info is gone. I have given detailed instructions for how to make both of these kinds of tea, so just click on the "All Hally's Messages" tab, and you will find them!