A lot of the decisions depend on your restoration intentions and budget - concourse trailer queen to cosmetic restoration to safe driver.
Are you looking for assembly-line correct and dated parts only, good used GM take-offs, or would reproduction parts do?
NOS parts often differ from Assembly-line parts in appearance and quality of fit.
There are plenty of parts vendors for reproduction parts, but there's no one-for-all supplier and it's best to shop around for regular special offers, research the best recommendations for each area of the car being worked on, and (if you can) go and look at the parts before committing to buy. Return policies differ and should be understood. A good starting point is to order catalogs (usually free) from the major vendors, then you can sit back and look through them and compare prices - a list of parts you will need can be very helpful, especially if you cross them off on purchase to make sure you don't forget a major item (and avoid buying more than one.....).
You may well have done this before, but if not, I would strongly recommend you have a plan for your restoration, and a timeframe and a budget in mind. Work out what you can do yourself, what skills you'd like to learn (or improve), and what you will farm out to a professional. Get a good set of tools, organise your work area and take lots of photos; documenting the tear-down as thoroughly as possible can be invaluable during reassembly, and can also help preserve or mimic original factory details.
