All the times I have visited the US, I have not been able to find any chocolate sold in convenience stores, supermarkets etc that doesn't taste like crap. The best I've found is a muskateer bar which is vaguely bearable. It's similar to the cheapest chocolate you can buy in the UK.
Perhaps if you go to a specific boutique chocolatier you can find some nice chocolate. But in the UK, you can just pop into any newsagent corner shop and get some good chocolate.