I made contact with Sonya Looney from the Ergon customer support. I asked her where it is possible to buy the Ergon grips end caps that seal off the tips of the handle bar.

Here is here answer:

“…They will be available on the website in about a month.  If you need some now, you can get non-branded ones from any bike shops, they just won’t be Ergon, but they will work…”

So thank you Sonya for the quick reply!