So your cassette is toast?
If you're ok with friction shifting, virtually ANY old 26" wheel off craigslist should work just fine, whether it be cassette or freewheel, 5,6,7 or 8 speed. That might be the cheapest.
But since you have a Uniglide cassette, there's a free way to get a "new" cassette! Just take the cogs off and flip them over. Sheldon Brown writes:
http://www.sheldonbrown.com/k7.html
Sprockets smaller than 14 teeth used a built-in spacer, but the other splined sprockets were reversible, so that if you wore out one side, you could flip them over and the other side was just like new! Sprockets with a built-in spacer were available in 5- 6-speed or 7- 8-speed (narrower) versions.