You do not need public certs for internal testing. Just install a Certificate Authority in your environment on a windows server (possibly even your test OCS Server). Then you can issue certs.
You also need to ensure that the CA Cert is contained in the trusted root store of the computer for each communicator client. With Windows CA, you can run http://caserver/certsrv (where //caserver is your CA server) to get this. you can then manually load it onto each client.