We've watched a few Rifftrax using the modern method: Download a snarky MP3 commentary track, put a standard DVD in the player, press start on the DVD and MP3 player at the same time, and enjoy. This method has some problems - speaker placement, synchronization, etc. But the golden advantage is: The riffers don't need to get permission from the movie rights owner.
For some movies, that's not an issue. There's a ton of "great" movies that have fallen out of copyright. You can find a lot of them in Archive.org, for ex., or as $2.99 DVDs in the cheapie bin. You can also find them with added riffing from RiffTrax, including RiffTrax: House on Haunted Hill and RiffTrax: Carnival of Souls.
I've mentioned House on Haunted Hill before; it was on a double-bill DVD with The Bat. I think I summed up the plot nicely there - Eccentric millionaire Vincent Price and his wife invited 5 strangers over for a party, offer them $10,000 if they survive the night, then give them a gun and lock them in. This is a great little Old Dark House story with one problem - some of the frights are exposed as frauds right off. If you've been spoofed once, you have to assume that anything and everything thereafter is a spoof.
The riffing on this is fine, but not overwhelming. I remember the movie more than the gags.
Carnival of Souls is a different critter. It is Herk Harvey's magnum opus. Harvey ("Herk the Jerk" to his kindergarden buddies) was a director of industrial safety films. This is his only attempt at feature entertainment. It is actually amazingly successful, if you consider the source.
A young woman is riding with a couple of friends, when a drag racing accident sends them into a swift muddy river. Only she survives. Who is she? A recent graduate of an organ school (the musical kind). And so she leaves for Salt Lake City, the very day after the accident. But even as she drives along the highway, the visions begin to appear, visions of ... the DEAD!
Our heroine is played by Candace Hilligoss with a touch of style - she isn't very concerned about the death of her friends, playing organ in the church is just a job, the creepy neighbor is just a nuisance, or even a welcome change from the ghouls she is beginning to see. It's an interesting characterization, and in response, her agent dropped her as soon as he saw the movie.
Best riffing - the riffers have written little songs to go over the DVD menus. Don't miss them!