how to select VM64 with VDOM to choose to replicate our network?

Dear all, We want to migrate our network from static protocol to dynamic proto like ospf. I have made some vdom in my network and I would like to know which version of fortiOS VM we will have to download in order to replicate a small scale of our network. So far we could not find the correct VM fortiOS on on the protal. (with vmware esx) Thanks for any suggestion