Either from Kozma, or from the Auction House, or by trading in /2 Trade.
Other players will generally give you good deals, such as selling slots for about 195 ce rather than Kozma's 250 energy price. They make a small profit margin since it's possible to use only 150 ce by buying on a full mist tank, and you can get the item without having to wait a whole day for regeneration. It's best if you check /2 before buying, if you're using cr. People will often give good deals there.