Dead Comment
No need to just stick with the classics, though - modern prog has branched out into so many different areas, there really is something for anyone who loves music out there. And it's also not necessarily limited to rock-oriented guitar driven music, there's "progressive" music being made in pretty much every genre these days, and a huge back catalog spanning decades to explore.
A couple places online that I've found great recommendations are:
* the /r/progressiverock subreddit (a little bit repetitive among the "classics," but still lots of good discussion and recs)
* MOROW - Prog rock radio, I've found a lot of cool stuff listening to this station
* The Best Radio You Have Never Heard - a somewhat eclectic but mostly rock oriented podcast, every two weeks he releases a 1-2 hour show which has lots of great music. This has been going for almost 20 years, and has been consistently excellent. One of my favorite things about TBRYHNH is that the host often finds cool "deep cuts" - live performances, B-sides, etc that you might not have heard before. This show is where I first heard Close to the Edge, it pointed me towards the excellent set of Yes shows that was released as "Progeny" a few years ago, the amazing live version of Time Passages from the Rhymes in Rooms album, Grey Cell Green by Ned's Atomic Dustbin, and dozens of other great tracks. Can't recommend it enough.