The CDX-1 required a special type of CD in order to record called "High-Speed CD-RW." These were different than the standard blank CD-R or CD-RW discs found in stores today.
CD manufacturers stopped production of these type of high speed CDs many years ago and finding them may be difficult. Since this type of "High Speed" CD technology is no longer manufactured in the computer or audio music industry, we no longer have resources for acquiring these discs. The only place to find "High Speed" CD-RW discs may be online (For example, searching for "High Speed CD-RW" in a Google search). Brands that were suggested were TDK, SONY, MAXELL, FUJI, MITSUBISHI, RICOH, HP, or ALERTEC.