Ready for planting and eating.
The item id for Seed Lettuce in unturned is 339. You can use this ID with the commands below to spawn the item, give it to a player or buy it on a modded economy server.
Useable: Barricade
Build: Farm
Size_X: 1
Size_Y: 1
Size_Z: 0.2
Health: 1
Range: 4
Radius: 0.25
Offset: 0.375
Growth: 10800
Grow: 338
Explosion: 43
Blueprints: 1
Blueprint_0_Type: Utilities
Blueprint_0_Supply_0_ID: 338
Blueprint_0_Build: 27
PlacementAudioClip: Sounds/FoliagePlacement.mp3
Has_Clip_Prefab: false