No idea about gardening, but they drop nicely from Gobs in Horutoto. If you're high enough level not to get aggro and kill them quickly (20ish I'd say, or 30 if you want /THF) then you can probably get plenty of them without breaking a sweat.
Plus you get things like Goblin Armor/Mail which can be desynthed into nice items if you have the craft high enough
