I suppose it depends upon how "really rockabilly" you are. Does it have to be countrified rock-n-roll, or is something more rockin' on your mind? Do you have a deep-seeded need to impress your friends with your knowledge of more obscure artists, or are some of the more popular folks dabblin' in rockabilly like Gene-n-Eddie and Buddy Holly acceptable to you? There's a lot of great tunes from the 50's that are rockin' but not 'billy so much. That's where the following companion sets come in.
In 1999, Rhino Records came out with Loud, Fast, and Out of Control: The Wild Sounds of '50s Rock. It has a lot of great tunes that fall outside what Colin-Escott wannabes would call "rockabilly." There's great r&b (Amos Milburn's Chicken Shack, Red Prysock's Hand Clappin') and rockers (Little Richard, Chuck Berry, Chan Romero, Ritchie Valens). But there's plenty of rockabilly as well (Joe Clay, Ronnie Hawkins, Wanda Jackson, The Collins Kids, The Johnny Burnette Trio, Carl Perkins, Jerry Lee Lewis, etc. I'll let you look it up on your own. As it's 9 years old, it may be harder to find these days, and it is a 4-disc set, so it falls outside the single and double disc treats out there.
In 2006, Rhino put out a sequel of sorts, Rockin' Bones: 1950s Punk and Rockabilly. It has terrific liner notes, as did Loud Fast and Out of Control, including a contribution from Slim Jim Phantom. Out of the 101 tunes, there are about 10 tunes that overlap, but it is definitely packed with songs that more strictly meet the rockabilly definition, and it has more lesser-known artists, like Hasil Adkins, Tommy Blake, and Ric Carty alongside the legends Eddie Cochran, Buddy Holly, Roy Orbison, and Jerry Lee Lewis. Both collections have secured Elvis tunes, which is impressive.
A more affordable British compilation is the 2-disc As Good As It Gets: Rockabilly. Because of copyright issues, there are Elvis recordings from the Louisiana Hayride rather than their studio counterparts. But I think it's worth checking out.
To dig a little deeper, I would definitely look at the Bear Family comps. They're always well-annotated and sound terrific.
Beware of the Buffalo Bop series. They have a ton of obscure tracks on their compilations, probably those whose licensing was never renewed. The few I own seem to bear out why these tunes languished in obscurity. And the sound is not good at all on some of the tracks as they're unable to get a good source for the recording. Spend your money elsewhere. Although if there are fans of Buffalo Bop records, I am open to a recommendation of any collections you feel are worthy.
All the best on your quest,
Rockin'-billy