The Internet Archive has a growing collection of OTR - old-time radio - material. Both Dimension X and X Minus One are there in their entirety. This means (I hope) that it is no longer to scour the web for these 1950s classics, or pay good money to traders to access this public domain material.
Suspense is also being collected on Internet Archive, but so far only one episode ("Riabouchinska") has been uploaded. This 1947 episode has the distinction of being adapted from a Bradbury story that was unpublished at that time; it later appeared in print under the title "And So Died Riabouchinska", and was also dramatised for the television series Alfred Hitchcock Presents.
I have updated my Audio pages to provide links directly to the Bradbury episodes on the Internet Archive, and will continue to add links for any episodes that become available in the future. I have also added links to other downloads currently available, although my experience has been thatMP3 links tend not to stay live for very long.
And in case you thought time travel was an impossibility, you should check out the Archive's fabulous Wayback Machine.
The Vix Audio Show podcast (#5) features the 1980s radio series Bradbury 13, and includes an interview with the producer Mike McDonough. You can read more about the series here.
Finally on the Bradbury audio front, Colonial Radio Theater on the Air have announced that they have recently completed production of the first audio dramatisation of Bradbury's Dandelion Wine, due for commercial release towards the end of 2006. They have also announced a forthcoming audio production of Something Wicked This Way Comes.