I buy all my Isuzu 6bd1 engine parts from Hamilton Engine Sales, Portland Or. They have been absolutely great to work with for me. Though I order through them, the parts come from Isuzu Industrial Engine parts suppliers here in the Midwest. Always have had very fast shipping, my last order of some gaskets and other parts came the very next day.....without additional shipping costs.
You'll need your serial number off your motor before they will start the parts process. The number is stamped into the block on the lower left (drivers side) side towards the front of the engine block. You'll probably need a wire brush and some spray cleaner to find and read the number. The numbers are fairly large though, maybe 3/4" or so. Find the number before you call and you're good to go. Depending on which sales person you talk to, you may need to remind them that they need to look in their "books" for the motor and not on the computer. Last time I called, the sales person was looking on the computer and couldn't find the motor until I mentioned that due to the age of our motors, they only had hard copy records. At least at that time, our motors were not on their computers.
Like I said, Ive always had great success working with these folks. I believe they can get anything and everything for our motors, these motors are still used in the fishing industry out there. You won't have much luck trying to get parts through an Isuzu "truck" supplier. Our motors are from their "Industrial" line and the two divisions of Isuzu don't play well together.