I don't know what they're made of. I see some say nylon, others silicone, others rubber. Some call them "O" rings. Some call them washers. Which is the best kind to get and why? And like for regular razors (like Razorock, Merkur & Muhle) where do you buy such things? Hopefully, they're someplace inexpensive like eBay. I need only about twenty of them. Links greatly appreciated. Thanks.