I see you are in south Orange County, so two ideas:
One, join the local OC chapter of the Buick Club, we are a fun bunch, and sometimes some good knowledge and parts in the group.
The other is Desert Valley Auto Parts in Phoenix area. Last I was there, they had a 53 Special, freshly arrived. They are not cheap for prices, but you can be there and back in a day - a long one, granted - and bring it home safely yourself...
Oh, and one more, Lamar Wilkins has a small private yard in Lancaster area. Private message me if you would like his number.
And as said before, it may be simpler to match your engine - a standard versus Dynaflow. I don't think they can easily interchange...