Zeintop went out of business, but another guy picked up the design and is making them. There's also another guy making Zeintop-style racing tops that (to me) are not the kind of top you want for street use.
If you find the Cayman-style top for the boxster, you're going to pay $2-4k for it. The factory ones that aren't as good looking are selling used for $1000 or so. I think Woody has one of those for sale (itsnotanova is his screen name).