A very interesting subject! Some folks refuse to drink tap water and prefer bottled products, but I've heard anecdotal studies that pour cold water on these habits! While some sources are well or spring, many are simply "purified" in plants and bottled in plastic. There are concerns with chemical leaching, particularly when heated in warehouses, transit, etc.
I used drink tap water and provide same for my birds, but my house plumbing has proven problematic. Very thin copper pipes coupled with local water chemistry caused a series of pinhole leaks, leading to tremendous damage on two occasions. Given the choice of a total repipe - expensive and very messy - I opted to reline existing pipes with a polymer substance that seals and protects. Yes, I did much research and properly applied is highly effective. The problem is it may or not be BPA-free. So now, I bulk-purchase water from a "water store" that is highly filtered and stored in BPA-free 5-gallon drums. One drum is in each bird location for easy distribution into their bowls. For family use, I purchased a free-standing heater/cooler unit that dispenses on demand.
I once asked my avian vet about the best source of water, and his answer was amusing. Essentially, most any good source is better than the "s**t water they drink in the wild!"