It seems like it’s hard to keep track who is actually importing them now but I’ve seen Nash Karting stickers on some energy karts around and Snow Racing in Sacramento has been a dealer/importer at certain times.
What parts are you looking for? The Energy kart that I had was pretty “European Generic” as to parts. I believe it was made by Parolin. Russell Racing at one time had a kart with the same braking system. Beyond that, I can’t think of too many parts that cannot be substituted.