Hello,
I had the same problem, even Amazon doesn't sell them any more. I contacted Petkit directly at support@petkit.com. They offered to send me the replacement pumps I needed for 11.90 USD each, including postage, on payment in advance via PayPal. I placed the order today, and the stated delivery time is two to three weeks. I'm not going to declare victory until they're actually delivered, but so far it looks pretty promising. I hope this information helps.
Best regards

I have never read that you can buy replacement pumps...