The Imperial Ornamental Tile decoration can be dropped from Stalwart Leader Command Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Stalwart Leader Command Pack, include a random selection of items from the Stalwart Leader Command collection of items. You aren't guaranteed to get an Imperial Ornamental Tile decoration from a Stalwart Leader Command Pack, but it is one of the few ways to get an Imperial Ornamental Tile deco apart from buying it on the GTN from another player.
If you're looking to get a Imperial Ornamental Tile, 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.