I buy my nipples where they are of good quality, and good price, and supply houses get the same ones without UPC tags, typically.
The best nipples are schedule 80 pvc and made in USA, and have machined ends that do not eat a washer.
No one has been able to tell me if they are suitable near a water heater, though one would assume so judging by the drain valve.
Brass is nice, but nearly all imported, and I find that the threads are way off spec, and often leak.
lately I have been using stainless steel, which I aquired a few hundred pounds of in an auction. Use tape and dope as they are so hard that they leak without godzilla to tighten.
Better to go to PEX 100% anyway, and slowly on the way.