I think you are talking about VCD (Video Compact Disk)? There are places on the web like coolvcd.com where you can purchase legal, studio authorized (non-bootleg) VCD's at good prices.
As far as quality, there are so many variables involved it's nearly impossible to answer your question, but in short, standard and low-bitrate VCD's (Mpeg-1) can look poor/fair to good, high-bitrate nonstandard (X)VCD's (Mpeg-1) can look pretty good. SVCD (Mpeg-2) can look pretty good to really good, but there are exceptions to all I've stated above as a lot of this depends on the source, encode, etc. Most large screen displays (not just plasma) will exploit any picture flaws.
If you are interested in learning more about the format, you might want to check out www.vcdhelp.com .
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