The Black Gourd
You can buy this at Cysero's shop at "Cysero's orb" (far left of the shop) for 50 DragonCoins.
If you don't have any DragonCoins you can get them from quests
One quest to get them from is "Circuit city". Go to the black smith's shop in FalconReach, and say "Quests" then "Circuits".
An easier quest to farm for gourds is the "Honey Quest". Go to Oaklore, then go left from the screen with Rolith
Then go in the door on the left side, the talk to the standing knight
Say "Honey Quest".
It is very rare to get any gourds, so be patient!
Some more places you can get gourds are "Voltabolt's Challenge", "Richard's gear"(a Popsprocket quest) There are more! Check the official forums for more locations.
(Note: The LAZIEST way to get it is from the reagent gathering quest from Reens. Start the quest, leave, repeat until you get the gourd you wish. It's a rare drop so it will take some time. You just have to repeatitively click the same place over and over...)