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I bought my last one from Advance Auto because of all the local ones available, it looked just like the A/C Delco one and I got a 1 year local guarantee. Since GM is no longer making the part, they "source" parts from other suppliers and slap their name and an extra 30% on it.
Rockauto is a great supplier but when you add shipping into the price, they usually are not cost effective. My Vette is also my DD so I rarely have the luxury of waiting for a part, lol. If I have time I look on eBay for parts sellers that are closing out stock. I bought a knock sensor for half price and an IAC at about half as well. Those were maintenance items so I had time.
Ditto ^. And don't forget Craigslist. Sometimes you can get parts that are 'laying around' for a decent price, but always remember the caveats especially for elect stuff.
Just had the master and slave replaced in my 86 convertible in August. Both came from Napa (around $50 ea) and both have work flawlessly for me once all the air got bled out of the line.