Looks like Natchez has #5,6,7.5 & 8 in stock if that helps. Not sure on price. I have an old 16 gauge H&R and plenty of ammo. My favorite shotgun caliber.If you intend this Seeet 16 as a shooter, to see regular use, consider the cost and availability of 16 gauge shells.
I traded into a 16 ga. semi-auto 2 years ago, and finding ammo has been a problem.