I have a rule about the Aprons. If they say they have it and bring me to it, I say, "Thank you," take it and go pay for it.
If they tell me they don't have it, I say, "Thank you," then go find it myself, take it and go pay for it.
The latter happens a lot more often than the former.
The bottom line is this for Korky products:
Lowe's and HD each have a decent selection of Korky flappers, but they vary between the two. Lowe's has a wider variety, while HD focuses more on the universal ones. HD also carries the Red "Fits Toto" 3" flapper that works on most GMax toilets and some EMax (I did a thread on the ones it fits), but it's available online more than in the stores.
EDIT: HD now carries the 528MP Maxperformance fill valve, both online and in some stores. Check the web site to see if it's in your local store. HD for some reason calls it the "528MCM" in its computer.
http://www.homedepot.com/p/Korky-QuietFill-MaxPERFORMANCE-Fill-Valve-528MCM/203145423
Lowe's has for a long time carried the 528MP Maxperforamce fill valve that Terry recommends so frequently.
http://www.lowes.com/pd_336988-868-528MPK_0__?Ntt=528mpk&UserSearch=528mpk&productId=3284948&rpp=32
(Lowe's calls it 528MPK if you want to search on their web site.)
Lowe's no longer carries the R528 replacement cap, but almost all HDs do, and HD will sell it online as well.
Lowe's carries the Korky Wax Free Gasket that replaces the wax ring in a toilet installation.
Lowe's carries the Korky replacement seals for Aquasource flush valves.
HD carries the MaxPerformance toilet plunger. We have a couple at home (for our older toilets), and they have a nice lip on them that really gets into the trapway. However, the rubber is thick and high-quality, so it resists pressure more than others. You have to be strong to use it best. Clorox used to make a nice plunger with a nifty holder; I use that one in the city. It doesn't have as good a lip as the Korky, but it's easier to push on.