Hi James, Welcome to the turbo Buick community. Other than noticing you posted this question in the "turbos, ect" section, I'm not sure what you are looking to change or how the car is set up now. One thing to remember is that there are vendors who are just re-sellers of parts and there are vendors who are actual fabricators and manufacturers.
The OPG catalog is nice to see a bunch of stuff for our cars but OPG does not make anything. They are a reseller and in many cases it is not clear what you are purchasing. Example is weatherstripping. OPG doesn't specify whose weatherstrip you are buying - is it Metro, SoffSeal, Steele, Fairchild, etc.
And there are vendors who cater more towards racing while others cater more towards restoration, stock appearing and NOS parts. So, I'll just post links to various vendors beginning with turbos and related stuff and let you explore their offerings.
http://www.limitengineering.com/
https://www.precisionturbo.net/
http://gpopshop.com/
http://www.fullthrottlespeed.com/
http://www.cottonsperformance.com/
http://rjcracing.com/
http://www.hartlineperformance.com/
http://www.taperformance.com/
http://www.kirbanperformance.com/
http://arizonagn.com/
https://www.gnsperformance.com/
http://www.gn1performance.com/
http://www.buickgn.com/
http://www.mikesmontes.com/
http://www.highwaystars.net/
I think this is a pretty good list of honest and qualified businesses to start with. As with so many things about these cars, parts combination is key. A bigger turbo may mean other changes are needed to complement that mod like fuel system stuff (pump, pressure regulator, injectors, prom chip, ect). But I'll leave things like you know what you want to do and how to do it, just not where to source what you are looking for.
If you are running the factory ECM and not a FAST setup, Eric Marshall at
Turbo Tweak is the source for prom chips among other stuff.