As Kyle said: there isn't any common, public way to search for publicly-available rkt containers in the way that you can do so with Docker Hub.
Part of the reason for this is that there is no corresponding `rkt push` command (yet). You _can_, however, use any Docker image with rkt. Since Docker repositories do not have the same verification capabilities as do the manifests for rkt, you generally have to disable such checks when using Docker images. The exception is for those Docker repositories hosted on Quay.io, which provides a rather flexible rkt-compatible, namespace-oriented verification mechanism.