I don't know. I ordered these headphones last year and nothing has broken yet. For spare parts, it seems to me that digitec is responsible - and apparently they have them now.

Update:
Die Earbuds sind ja jetzt im Digitec-Galaxus-Sortiment! Siehe Link:

Logitech Earpads f/H570e/H650e/H820e

I also had the same problem in 2018. My enquiry at the time with Digitec/Galaxus customer service resulted in the following response:
"...After consulting with the responsible department, we received the information that the matching ear pads are currently not available from us. We also do not receive a delivery date here. You can find the product here:..." and then a link to eBay (!) followed, which unfortunately points to nothing today.

Unfortunately, this is not sustainable. At the time, I wanted to buy a headset with replaceable ear buds so that I could use the product for as long as possible. But if the manufacturer does not provide any replacement earbuds, the interchangeability unfortunately makes no sense at all... :-(