Roger at Lensrentals.com has completed his bench test of the Canon EF 16-35mm f/2.8L III wide angle zoom lens. It looks like the lens performance is the new pinnacle for wide angle zoom lenses and you should probably go ahead and buy one!
This summary is quick and simple. From a resolution standpoint, the Canon 16-35mm f/2.8 Mk III is the best f/2.8 wide-angle zoom available. You might be better served with the f/4 IS and some money in your pocket. There are also some very good wide-angle f/2.8 zooms available from third party manufacturers that are a lot less expensive and might offer more bang-for-the-buck. But if your style of photography needs the highest resolution you can get with a wide-angle lens, well this is it. I don’t use a wide-angle zoom all that often, but when I do, it will be this one. Read the full review
It looks like the EF 16-35mm f/2.8L III is in stock across most retailers.