ID Parts has the gaskets and o-rings for the injectors. Not sure on torque but it's a skinny bolt into aluminum so not too much.
Check that the plastic injector housing isn't cracked and keep them labeled as to which cylinder they came from. They're tuned to each cylinder for some reason. There's a sticker on the timing cover somewhere relating to which injector belongs to which cylinder.
Also be careful of the little plastic t-fittings on the return lines atop the injectors. They break very easily. Duramax diesels use the same sort of hoses and "t"s for le$$.
There's a special injector removal tool you probably won't need. I used an open ended wrench, patience, and some small wood blocks as fulcrums.