The valve cover seal is made with an o-ring type design and can be reused if in good shape. If it looks flattened, brittle or currently leaks, replace it.
Once the glow plug is removed, you might find the manufactures name and model # printed on them, in the event you want to look for an alternative source.
Testing the relay is not enough diagnostics. Verify there is power at the glow plugs. Glow plugs are fairly robust and they take a fair amount of current to operate. Although a pain to do, I'd clean (bare metal to bare metal) and inspect every electrical joint in this circuit including the wire crimp itself.
2012 JD855D OPS Black Poly Roof, Deluxe Cargo Box, Front and Rear Protection, Power Lift, Front Hood Rack, Rear Screen (homemade), 800 Watt Inverter, 2500lb Winch installed in cargo bed, Front and Rear LED auxiliary lighting (4" square 27 watt), Front and Rear CV Guards, Koplin Side Mirrors