Can you tell me the size of the shower chair tips? I don’t see a model number, so I’m not sure these will fit.

These shower tips will work for this model of chair:
https://carex.com/products/carex-adjustable-bath-and-shower-seat-with-back
If you have an older model or chair, please reach out to our customer service team so they can assist you: https://carex.com/pages/contact-us

I just need the seat replacement, how can I order that?

Unfortunately, we do not currently offer a seat replacement.