If that seal is at the interface of the injector and the manifold, then it is a critical part because air will leak around it. I don't have a diagram of the intake to know if that's the purpose of this part.
Injectors are everywhere, so I wouldn't buy one from the dealer. Junkyards, discount auto parts stores, and the internet are flooded with them. There are 10's of millions of these, so everybody has them new, used, and reconditioned.
There is something else you could try: an O-ring. I cannot say for sure if this will work, but other vehicles on which I've played with the fuel injectors used simple O-rings to seal the injector at the manifold. Auto parts stores have O-rings, as do most home improvement stores or hardware stores.
Caraturd3: 2016 DGC AVP, 4.5k miles.
Caraturd: 2000 DGC Sport, 126k. My first minivan, with badly rusting strut towers that ended its life prematurely. Unceremoniously junked on 7/23/2013.
Caraturd2: 2006 DGC base, 65k. Traded on 5/31/2016.
2015 Ford Focus ST3
2003 Ford Focus ZX5, 5-spd. My winter car because the ST doesn't have winter tires.