Runnings and Walmart still stock 16ga. shells, I was looking at them today. I love the 16ga., but for most people it's either equaled by modern 20ga. shells or exceeded by a HUGE selection of 12ga. shells.
The last listing I saw for a new 16ga. was just before New England Firearms went back to being H&R.
I don't shoot my 16s to full potential, I just shoot low brass 1oz. #6s, which are more expensive than their 12ga. counterparts. I actually see the 16ga. going the way of the 8ga., like the 10ga. is now going.
I mean in all reality, all you need is a 12ga. with a 3.5" chamber......and you can shoot 1.75" mini-shells all the way upto 3.5" magnum loads....and everything else is cancelled out.
But trust me, there are a lot of nice older 16s out there still. I have 2x doubles in 16ga. and I still shoot them.