Help! Natursutten bottle nipples sticky-alternative that fits?

Okay so the bottles we use are natursutten (pictured) and we love them except for the fact that the nipple after multiple uses start to get sticky! Is this a thing with natural rubber nipples? I’ve never seen this before and I was a nanny for years and years. It starts to stick after 2 months of use.

I went to buy replacements and they are out of stock everywhere. So my question is, has anyone used these bottles and use a different brand nipple that fits them? Is this even possible? I would hate to buy new bottles just because the nipples being out of stock. If you have had success with another nipple to fit in the narrow glass bottles I would love to know the brand. Preferably rubber or silicone (low tox)- thanks!!