I doubt that SI would sell the seals and honestly, if you get a good enough collection of them for use; you will find that may have an extra when the time comes anyway.
I have a collection that leaks halfway down or lower but it only leaks while vaping so if I keep them horizontal and only covering one wick, it usually stops. Re-filling it will also stop the leak but the trick works so I typically just do that.
A suggestion if it matters, when you have one halfway down and if you have a selection of juice - try adding another flavor and turn it upside down and right side up a few times to mix it. You may find you like the mix.
Also, and I didn't see this noted but I could be wrong......you can only re-fill them with juice if the tip of the needle is above the juice while upside down. You will just push juice out the other side if not......and remove the needle while it is upside down. sorry if that was repeated.