http://www.tvnihon.com
Best place to get Full Kamen Rider series from Faiz (aka 555) forward. Each series is also in it's own universe except for lately where they allow the previous rider to show up in the movie to help out. They offer torrents as well as direct downloads. The direct downloads are only available on the forum, while the torrents are on the Torrent Tracker page.
Best series to start with would either be pretty much any of those from Faiz forward. I started with Faiz ... but others like Blade, Kabuto, Den-O, Kiva, Decade, W (pronounced 'Daburu') or OOO (pronounced 'Ozu') are fantastic as well. It's hard for me to choose a few favorites and tell you just those, though. I never really did get into Hibiki (which was between Faiz and Blade), but some people like it.
Enjoy your Kamen Rider and stay away from the American Adaptation of Ryuki known as Kamen Rider Dragon Knight. It was terrible. Go find the actual source series, Kamen Rider Ryuki, and you'll enjoy it more. I can't remember where I found it, though.