Yeah, I've substituted before, too. My plan of attack usually is to verify that they have everything I need before they get my grain bill together. (I don't have any ingredients at home.) If they're out of too much (or anything), I usually have a backup beer I wouldn't mind brewing. But I wanted to brew an Oktoberfest, and for timeliness reasons, I substituted.
I had forgotten about stating my disappointment until I went in there the next time for a brew day. It was weird, because the manager approached me and said, "You were in here a couple weeks ago about some Oktoberfest yeast that we didn't have." I said, "Yeah, it's no big deal. I used S-23. I've sampled it and I like it." I thought he was going to apologize for being out of stock with it. He floored me when he told me that I offended the employee. I wasn't belligerent, wasn't cussing. I wasn't even raising my voice. My usual comment to a store that's out of stock of a product I want is something like, "Tough to sell something you don't have." I probably said something like that. Did I mention it's the only store within 50 miles of my house?