Is there a cert you are going for specifically?
I've never tried it but it looks reasonable. It is paid, but it offers courses focused towards certifiacates and hands on labs, with fairly reasonable pricing. There is a a weeklong free trial, so you can just use that to test.
Also check out these:
https://training.play-with-kubernetes.com/
They are truly free, but have a lot less content. I like to use them to teach stuff sometimes.
You can also set up a homelab with Kubernetes for hosting stuff. I actually really like Kubernetes for that usecase.