Ammo in any common caliber not to be found on shelves. Most FFLs hold for their best customers. Can still be found online if you are willing to pay, although perhaps not your preferred brand or model www.ammoseek.com seems to work well for my friends who didn't stock up or have purchased their first firearm.
I would second ammoseek. I used to go to ammo.com or sgammo.com, but in the current climate both of these are out of stock on almost everything. I used it for the first time last week looking for .223 in 38 - 40 grain (kind of rare compared to the usual 55 grain and up) and found some. Prices, though, are not great.
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