Remington and possibly another manufacturer started making 16 gauges again.
http://www.shootingtimes.com/longgun_reviews/rem_16_0507/
So, you should start seeing a few more places to buy ammo from, too. Otherwise, Cabela's and some other mail order companies sell 16 ammo through the mail.
I have a "sweet 16" Browning auto I bought from an officier, when I was stationed on the West coast in the Army over 30 years ago. His wife had used it for shooting clays. I used it for all sorts of bird hunting for a number of years, until I bought my first Benelli 12 a few year back.
The Browning 16 is really a beautiful Belgium made shotgun and I plan to hold onto it.