Thanks--makes life easier!
I found DV Warehouse has a number of G5 logic boards but none had the exact model number:
They have a toll-free number. You could ask if another board # would be compatible.
Another option is to contact recylers or schools shucking off older gear. Using the enigma of " A used computer is worth more parted out," you might be able to pick up another G5 cheaper than its LB. Unless the LB had the dreaded "leaky capacitor" failure, most computers don't get discarded for a bad LB.