Voted for Snikers, 5Stars and Boost aren't really widely available in the US you probably have to go to a specialty food store that sells British food to get them. Ever since Cadbury sold Dr Pepper/7up to Snapple they really don't have a large present in the US anymore. My favorite somewhat obscure candy bar (even though its made by a major brand Hershey's) is Whatchamacallit.
https://en.wikipedia.org/wiki/Whatchamacallit_(candy)