Trade resources are defined in ImprovementDefs.XML; search that file for <ImprovementType>TradeResource</ImprovementTypes>.
UltraSpice, EpimetheusPollen, HeliosOre, ThuliumCatalyst, TechapodHive, ArtocarpusViriles, MonsatiumDeposit, CrystallizedElerium, XanthiumDeposit, AurorusArboretum, PromethiusStone, HyperSilicates, PrecursorNanites, HarmonyCrystals, and SnugglerColony are the internal names of the trade resources.