I would say RiffraffDiesel.com, DieselORings.com, and K-MansParts.com all carry the OEM Gaskets, UVCH's, Glowplugs, and pigtails (those are aftermarket Ö factory had repair pins and connector shell pieces to repair the originals). Riffraff probably has the lowest prices of the 3 (but shop around for better prices). Parts:
Glowplugs: Motorcraft/Beru ZD-11, Ford F4TZ12A342BA
Gasket: Ford F4TZ6584A
UVCH: Ford F4TZ9D930K
Pigtail: Alliant AP0015
Glowplug Relay: I don't use factory as there are much better options: White-Rodgers 586-902 or Western Plow 56131K-1
Here are some testing procedures I use:
For checking the injector/glowplug wiring: From each valvecover gasket connection's outermost pins (2 per connector) to ground you should get 0.1 to 2 ohms (indicates good plug and wiring). From each valvecover gasket connection's center pin to each immediately adjacent pin, you should get around 3 ohms (indicates good injector solenoid and wiring). You should not get any continuity from any of the outer pins to the 3 inner pins (indicates no shorts between injector and glowplug wiring). You should also get 0 to 1 ohms from each of the external harness connectors outer pins back to the Glowplug Relay's large terminal on the GP side (indicates good wiring from external connections back to the relay).
To check the glowplug relay (GPR), measure the voltage drop across the GPR's large terminals. While the GPR is active (up to 1.5 to 2 minutes after the key is turned to Wait-to-Start) put your meter leads on the large terminals (one lead on one large terminal and the other lead on the other large terminal). The measures how much voltage is being "lost" across the relay. A reading of 0.3V or more indicates a bad relay. Also, check the relayís control wires (smaller wires) disconnected from the relay for battery voltage at the Red/Light Green striped wire and ground at the Purple/Orange striped wire (check both when the key is turned to Wait-to-Start).
Also, here's a link to the GB Remanufacturing Tech Bulletin for checking wiring from IDM to GPs: "GB Tech Bulletin #103: Ford Injector Driver Module (IDM) Replacement"