It used to be impossible to find them in the U.S. When I was little, it was a big treat to get just a handful from the Chinese grocery market. They were very expensive and not very fresh.
The best I've had are still the ones grown at my family's home in Taiwan. One of my favorite childhood memories is of my dad climbing up one of the huge lychee trees there and bringing down buckets and buckets of them. We'd just sit there for hours and hours doing nothing but eating lychees together.