There are ships that are rare or unique, of which DCs do not exist (anymore or ever did). I believe it will be possible to R&D most, if not all, of these ships later on.
Ships that you can build will have a faction affiliated. Example: YT-1300
. Incom Corporation is the faction you need to contact to have the DC assigned so you can produce it. See the Production section of the rules.
Ships that you can't build will have no affiliated faction. Example: Starlight