I have a Remington 1100 in 16 gauge that was given to me brand new by my grandpa when I turned 9 for duck hunting. It's killed a lot of ducks, squirrels, rabbits, quail and wood****. My older brother has one too. Ours are 48 years old this month and look like they are still brand new. My cousin has been after my brother to sell his and he keeps saying no, finally to shut the cousin up he said he could have it for $500,000. The cousin finally got the hint. 16's are awesome and I see them every now and then. I screwed up last year and missed out on a model 12 Winchester in 16 gauge because I hesitated for one day. I occasionally see them at Cabelas and some of the bigger gun handling pawn shops but it's not too common. I can't say I recall seeing an o/u in one though, usually pump is the config I see them in. I know Gallensons has a lot of used guns so you may want to try there too.It's a shame the 16 gauge isn't more popular. I think it makes one heck of a upland and waterfowl gun. The cost of shells are so dang pricey though. Good luck in your search.
You will find umpteen more options with the 20. Might I recommend getting the 20 in 3" magnum. That will give you close to 12 gauge ability when shooting the magnums....and you can still shoot the 2 3/4"I may just have to settle on a 20 gauge if I find the 'right' one.