I looking for someone that knows about A 500EFI ecm model 840136T2. I have a set-up but no harness or ECM what are my other options?

Aftermarket, probably easier to tune

Yep, theres virtually no mefi3s to be had left out there. or virgin harneses, if you find one, buy it! A used mefi3 programmed for a 500 EFI is 1500$ these days and most harnesses are hacked up or few and far between. Take that 2000+ $ your gonna sepnd on used electronics anbd buy new, Smitty