You don't necessarily need a new Delco unit. A rebuilt one from Auto Zone, Pep Boys, Kragen, NAPA, etc... will work just as well.
As for what caused the problem in the first place, that's difficult to say. If you KNOW that you didn't accidently reverse the positive and negative cables (even for just a second since that's all it takes), then you have to start looking elsewhere. Although not very common in the way of popping diodes, it "could" be the regulator. But really, that's about it.
Did you have the 3 shops check the diodes?