They are not buildable because they aren't really a "main" ship for the CIS
Most, if not all unique units the subfactions have access to are obtainable via mission rewards. Alot of them are available as starting forces as well, since for the most part they are auto-generated at the start of a GC.

List of everything the CIS can get from mission rewards in 1.4:
TRADE_FEDERATION =
MAF_Group
MTT_Company
Neimoidian_Guard_Squad
Sabaoth_Frigate
Sabaoth_Hex_Deployer
Auxiliary_Lucrehulk
Sabaoth_Destroyer

TECHNO_UNION =
Marauder_Cruiser
Geonosian_Cruiser
Skakoan_Combat_Engineer_Squad
Super_Tank_Company
Storm_Fleet_Destroyer
Recusant_Dreadnought

COMMERCE_GUILD =
GAT_Group
CA_Artillery_Company
Persuader_Assault_Company
Kaloth_Battlecruiser
CIS_Dreadnaught_Lasers
Providence_Dreadnought

IGBC =
Interceptor_Frigate
Action_VI_Support
Hardcell_Tender
Lucrehulk_Core_Destroyer
Auxiliary_Lucrehulk_Control
Storm_Fleet_Destroyer

CIS =
B1_Droid_Marine_Squad
Nimbus_Commando_Company
B2_RP_Droid_Company
CIS_Super_Tank_Company
Storm_Fleet_Destroyer
Protodeka_Company

PDF =
CEC_Light_Cruiser
Storm_Cloud_Car_Company
Customs_Corvette
Light_Mercenary_Team
Mercenary_Team
Mandalorian_Soldier_Company
Mandalorian_Commando_Company
Neutron_Star_Mercenary
Ailon_Nova_Guard_Squad
Sun_Guard_Company
Space_ARC_Cruiser
Auxiliary_Lucrehulk_Control
Elite_Mercenary_Team