First of all, I've always tried to avoid big box for plumbing supplies simply because they will always be out of something you need, and if time is money, you end up blowing the savings on second or third trip somewhere else.

I am now in Germany where we had basically five big box hardware chains. Now we have two. All of the little stores where you can get specialized stuff are long gone. Need a fine thread 10mm nut? You can forget it. Go to OBI (the German HD) for a few plumbing washers? You'll need to buy an assortment bag for ten bucks which maybe has two rings in it which are of any use to anyone. I once told a cashier "if this little bag is really worth ten bucks, then a car tire would cost more than a car."

But, Horror vacui (nature abhors a vacuum). Because of the market conditions, thousands of little Ebay stores have popped up providing us with what we need, and at reasonable prices. I now rarely go out to get things made of metal or plastic. The postman brings it to me. You guys have McMaster, which is great (no equivalent in Germany), but the model is still rather specialized. Partly, I suppose, because of high shipping costs, slowness, and unreliability of shipping in the US, and don't exclude car culture. I have plenty of friends who don't even drive.

When planning your next project, whatever it is, consider ordering all of the parts online. You need to plan a bit more carefully, but you might save some money, and especially lot's of time!