Well,
CD Universe has pretty good sound samples for every imaginable artist. You could try the file-sharing route [although w/ Prog Rock I doubt you'll have much success] or you could check out the various pay-for-mp3 services around, they usually have pretty good high-quality samples.
And if you're ever again in the mood for some new Prog, there really is no need to go anywhere else than the famous
Gibraltar Encyclopedia Of Progressive Rock for all the progressive rock info you could ever need...
...found here:
http://www.gepr.net/