Hook: Floor Small (Green Square)The Flag: House Ulgo decoration fits into a small green square floor hook.
Hook: Floor Medium Narrow (Blue Rectangle)The Flag: House Ulgo decoration fits into a narrow blue medium rectangle floor hook.
How to Get a Flag: House Ulgo Decoration
Cartel Pack Decorations
The Flag: House Ulgo decoration can be dropped from Architect's Stronghold Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Architect's Stronghold Pack, include a random selection of items from the Architect's Stronghold collection of items. You aren't guaranteed to get a Flag: House Ulgo decoration from a Architect's Stronghold Pack, but it is one of the few ways to get a Flag: House Ulgo deco apart from buying it on the GTN from another player.
If you're looking to get a Flag: House Ulgo, 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.