The Lowe's a 1/4 mile from my house used to have one of the largest selections of brass plumbing fittings,but they keep reducing inventory--anything that isn't a hot seller doesn't get re-ordered--also they don't carry any 5/16" compression unions that I like to use on transmission cooler lines--the person running the plumbing section says 5/16" is "automotive" and isn't used in household plumbing,so they dont carry anything in 5/16" like copper tubing and other fittings..
Home Depot has two stores ,both are about 6 miles away though,so I go there only if I have too..they may have more fittings than Lowe's..
Lowe's now carries some weird type of compression fittings that don't use two normal looking ferrules too,only ones in 3/8" and 1/2" and 5/8" for copper pipes is all they have now for the typical ones..
There are at least half a dozen well stocked old fashioned hardware stores in the next town over,but they are hit & miss as to whether you'll find what you need at any of them,and I have wasted an afternoon trotting from one to the other trying to find fittings,or enough metric nuts & bolts to assemble something..and their prices are butt rape..
It ticks me off having to drive around all day just to get $10 worth of hardware..only 2 stores have a wide selection of metric bolts and enough of them to let you finish the job..
I buy nuts & bolts at Tractor Supply by the pound,but they do not have much for metrics in stock,never the right length or pitch thread I need,or enough of them anyways..it's 9 miles to the nearest TS store..
I buy up every bucket of fittings and bolts,nuts,etc at the flea market I come across..most of the time my garage has more hardware than some stores do..but I have hardly anything metric..