When I got my Spectravideo SVI-328, it came with a floppy drive but no floppy discs. A chance visit to the Spectravideo Facebook group for a different matter led me to a cassette file that I could use to create a disc BASIC boot disc (and CP/M boot disc).

This video runs through the process of creating the disc BASIC disc using a SVI-CAS digital playback and recording device (the process to make a CP/M boot disc is almost identical).

Spectravideo Facebook group

SVI-CAS (pricing and contact details on the last page)