I think Sun still makes them. Rivendell (
www.rivbike.com) had them in the catalog a year or two ago. Not sure if they're still there.
Now that I think of it, was there ever a five-speed cassette? I think maybe the world had changed to six or even seven before cassettes came along, and what Riv had was five-speed freewheels.
You might also check Loose screws in Oregon (loosescrews.com, I think). And in a pinch, check thrift shops. Around here you can get a complete, running, fairly decent old bike for $25-$50. I've cannibalized many a Salvation Army frame building beaters and commuters for the family.