Hook: Floor Small (Green Square)The Office Chair decoration fits into a small green square floor hook.
Hook: Floor Medium Narrow (Blue Rectangle)The Office Chair decoration fits into a narrow blue medium rectangle floor hook.
How to Get an Office Chair Decoration
Cartel Pack Decorations
The Office Chair decoration can be dropped from Ultimate Cartel Packs, which are from the the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Ultimate Cartel Pack, include a random selection of items from the Cartel Market. You aren't guaranteed to get an Office Chair decoration from opening a Ultimate Cartel Pack, but it is one of the few ways to get an Office Chair deco apart from buying it on the GTN from another player.
If you're looking to get a Office Chair, 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 for direct sale on the Cartel Market, you could try buying Cartel Packs from the GTN and hope you get it.
Some items do rotate on and off of the Cartel Market for direct sale, and may be available later in the year, while others are almost never for sale for direct purchase. There is no way to know when certain items may become available on the Cartel Market, or when they might go on sale.