These are typically just rubber washers. The industrial supply places (i.e. McMaster) carry many sizes in a few different materials. I would probably go with Silicone or Neoprene. If you clean it really well before installing it, I don't think there will be any flavor issues even without an FDA approval on it.
If all else fails, I'll bet you can make one yourself. You can buy sheets of FDA approved rubber from McMaster and cut the washers yourself with a good sharp Exacto knife. I have done this for other brewing equipment like my mash tun.