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