The price of Curtas seems to have gone up over the past 18 months. eBay has plenty of listings for Curta I and IIs, but you'd have to shell out more than US$1000 for one, unless the unit was in bad condition.
However it's kinda iffy buying one that you can't actually test first. It's been too long since I've purchased anything on eBay so I don't know what the return policies are.
The guy at the show mentioned "always test the _one_ slider. Those go first"
Edit- spelling