The best way to contact writers (the ones that are alive, that is) is to contact the publisher of their last book.
You can also do some web searching for whomever you are wanting to contact. I've contacted a couple of semi-famous folks from email addresses I've found on the web, including one of the two people you mention.
Here's a clue: From the web, I found out that Gary Snyder is a professor at UC Davis. He was very kind in his response to the email I sent him -- but I was wise, asking him simple and direct questions for a specific purpose that was fully explained.