Some do.
No you can not sell REAL bullets in a gift shop. But, bullets are sold at gun shops
Try gun shops, pawn shops, want ad, for sale ad, gun shows.
It can go by whichever name the owner puts on it. You have dedicated gun shops, pawn shops, chain stores which sell firearms, online retailers, and, since gunsmiths are required to have an FFL to be able to legally operate on firearms, a lot of them sell guns, as well. For online retailers, the first two which come to mind for me are AIM Surplus and J&G Sales. Most gun shops with a storefront nowadays also have a website or may sell items on an auction site like Gun Broker or Auction Arms. For a listing of gun shops in your area (which was not specified in your question), a search engine would probably be the best option to find them, or you can look in the Yellow Pages.
Manufacture don't usually sell to the general public. However there are several air gun repair shops that deal with gun barrel manufactures. These shops are individually owned and you can try one of them to see if they will sell you a barrel. (See the links below) Also if your air rifle is a Crosman then they do sell individual parts for current models, They own Crosman, Benjamin and Sheridan. They have a web page to locate air gun repair shops for older models on their web page.
Gun shops, gun shows, garage sales, pawn shops, homes, etc.
Gun shops, pawn shops, gun shows, direct from distributors...
Books, gun shows, gun shops, pawn shops.
On line auctions, gun shops, gun stores, pawn shops
I'm not sure, but it might be those 5 pound Hershey's bars they sell at Hershey's chocolate world and in the little shops outside of Hersheypark.
Gun shops, pawn shops, gun shows, want ads.
The Blue Book of Gun Values can help. More current information as to what it might sell for can be found in online auctions. Local gun shops and gun shows can also be useful.
Possibilites include, gun shows, gun shops, want ads, internet gun shops.