The Ambient Machine: Waterfront decoration can be dropped from Ultimate Cartel PacksUnderworld Alliance Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Ultimate Cartel PackUnderworld Alliance Pack, include a random selection of items from the Ultimate Cartel Underworld Alliance collection of items. You aren't guaranteed to get an Ambient Machine: Waterfront decoration from a Ultimate Cartel PackUnderworld Alliance Pack, but it is one of the few ways to get an Ambient Machine: Waterfront deco apart from buying it on the GTN from another player.
If you're looking to get a Ambient Machine: Waterfront, the easiest way to get it is to head to the Galactic Trade Network (GTN) in SWTOR and buy it with credits from another player. If it's very rare and not available on the Cartel Market, you could try buying from the GTN and hope you get it.