As for used ECM/PCM I went through an online salvage yard (can't remember which one). Searched the page until I found a pickup identical to mine (same year, eng, tranny, everything) and bought both ECM and PCM. I figured if they didn't work I could get them re-flashed.
Long story short I hooked both computers up and they worked perfect. I'm now running with those two and keep the OEM units as my backup. I think the trick is to find an absolutely identical match to your rig.