Hi, Mark! Hi, Yalini!
We do not presently have a public API directory that you could use to programmatically discover live Millennium clients. However, with Meaningful Use Stage 3 compliance becoming a requirement for US providers in a few months, virtually all of existing Cerner clients are connected to the API or are finalizing the steps to set up the connection.
As far as Sandbox is concerned, we have our developer documentation available on https://fhir.cerner.com/millennium/dstu2/
including the self-service portal (green button, upper right corner) to register and test apps against a live Millennium environment. Developer program information is available at https://code.cerner.com/