Anyone with access to a computer can get 16ga ammo.
Off-the-shelf is splotchy, but some WalMArts, KMarts, Dicks, BassPro still carry some.
There's plenty of choices in premium hunting stuff; plinking/target, not so much. But Winchester, Federal, Remington and Fiocchi all make a 1 ounce load of #7.5 & 8 @ 1165-1200 fps that will handle all your target needs. You'll rarely see any < $6/box these days and reloading makes more sense than ever, especially for folks wanting 7/8 ounce for lightweight guns.
Beware of that Rio stuff. One of their loads in 16 that is most often found on dealers shelves is 3-dram equivalent, Heavy Field Load. If you aren't aware of it, it'll set yours ears back when you shoot it, especially in lighter weight guns such as the Ithaca M37 or SXSs. If you reload, try Federal/Estate ammunition as those cases generally will give 3 reloadings.