Honestly I've never ran across a real Turbo Buick specialist.
My go to guy is an ex co-worker and a great mechanic. He bleeds Mopar, but recently picked up his first Turbo Buick almost a year ago. He's been bitten by the intercooled Buick bug.
The only place I know of is PTS out of the Sac area. I've personally never been there, but I've heard several good things about it from several peeps.