The Greysheet, Coin Dealer's Newsletter, and Red Book are very popular. Are they available in online versoins?
Grey Sheet is easily purchased online. I purchase one every few months to keep an eye on prices of my favorite series. They do a very nice job of making them available online. You can get Blue Sheet and their other pubs online as well.
Coin Dealer Newsletter IS the Grey Sheet. You can order in anyway you want it. It's fast, easy and very convenient. http://www.greysheet.com/Commerce/scripts/customer_pdf_login.asp?svc=CDNi&
The Red Book is not on-line, but go for the big bucks and buy a copy. The info is worth the price of admission.