Can I buy replacement silicone septa for my Carousel PTFE caps?

Yes. The part number for the silicone septa (Suba-Seals) in the white PTFE Carousel caps is RR98076 for a pack of 100.

These are considered consumable items as they are likely to deteriorate over time with normal use, e.g. due to piercing the septa with a syringe for additions.

Suba-Seals are available in Viton if preferred; the part number is RR98176 for a pack of 100.